I believe (but have no way of confirming) that ReferenceManager can export an RIS file, which Zotero can import. Alternatively, look for BibTex or Endnot/Refer/BibIX export which it MIGHT have. Zotero also supports MODS, MARC, and RDF import, but I doubt ReferenceManager can export to those formats.

Please report back here with the format or formats you can/can't export to help other users.

Re. (2): RIS is supposed to be plain text. So it sounds like a non-standard export from RM to me...

There is some movement on these issues here and here in the development forum.

Consult your RM documentation or the technical support that exists for it: it should be able to import RIS.

How many references are in the file your trying to import? Try breaking the file into smaller chunks.

Alright! It was that! :oD Such a simple thing and I didn't figure out. Thanks Tjowens! It seems I can import citations in packages of 100. I get mistakes with accents and special characters but that's another thread...

MANY THANKS! Dani.

I don't see why it should not work, what is your RIS output? When exporting from endnote it works. check here for discussion:
http://forums.zotero.org/discussion/5311/importing-endnote-libaray-including-pdf-attachments/

SUCCESS!! I have had success with exporting from Reference Manager including with the PDF link problem. I have a large number of PDF files that are saved on my hard drive and in reference manager I access them from the 'link to PDF' field.

Steps:
1) export files from reference manager as an RIS file (highlight records /file/export select RIS)
2) open the RIS *.txt file that you just created in e.g. word notepad
3) here is an example of one of my references in this file:

TY - JOUR
ID - 2
T1 - ABC of diseases of liver, pancreas, and biliary system. Chronic pancreatitis. [Review] [3 refs]
A1 - Bornman,P.C.
A1 - Beckingham,I.J.
Y1 - 2001/03/17/
N1 - 11250854
RP - NOT IN FILE
SP - 660
EP - 663
JF - BMJ
VL - 322
IS - 7287
AD - University of Cape Town, South Africa
L1 - C:\References\ABC of diseases of liver pancreas and biliary system Chronic Pancreatitis.pdf
ER -

4) note that my PDF is under the field L1 and that it has some junk characters at the beginning and the end
5) when importing the RIS file into Zotero this causes problems. To get rid of these junk characters, cut and paste the whole exported file from notepad and into word (note that the junk characters are still there. You should be able to find and replace these but I was not able to cut and paste these characters into the find field of find and replace. However I found that if I cut and paste this back into the note pad, the character disappeared, however it was replaced with a space. To get rid of the space, you can e.g. find and replace " C:\Ref" with " C:\Ref" and then repaste this into the notepad file which you then save.
6) import this scrubbed txt file into zotero and it works. The PDF links are now in the URL, when you click on the URL it takes you to your record on your hard drive.

I just imported a large Reference Manager database into zotero (2.0.9). The problems I faced were:

1) Special German characters (äöüß...) got lost.
2) as described above, RIS export files contain non printable characters [\x00-\x1F], mainly arround the .pdf link, but also in other places...
3) a export file named <something>.ris hangs up zotero. <something>.txt works well.
4) I wanted the PM link to be a clickable web link in zotero
5) Editors are not imported as such

My solutions:

1) specify IBM-850 as input character set, or convert the RIS file to UTF-8, and tell zotero that the import character set is UTF-8. The automatic recognition only worked sometimes...
2) remove all non printable characters except CR. It has a special role in the "N1 - " record.
4) change "UR - PM:" to "UR - http://www.ncbi.nlm.nih.gov/pubmed/"
5) change "A2 - " to "ED - "

This can be done by hand with notepad++ for example, or by the following script run under cygwin:

---
#!/usr/bin/bash
#usage cleanup.rc infile > outfile
#
iconv -f CP437 -t utf-8 $1 >tmp1
#
sed 's/[\x00-\x0C\x0E-\x1F]//g' <tmp1 >tmp2
sed 's|UR - PM:|UR - http://www.ncbi.nlm.nih.gov/pubmed/|' <tmp2 >tmp1
sed 's|A2 - |ED - |' <tmp1
---

This way all .pdf files which were linked in Reference Manager will be copied into the zotero storage, which is what I want.

So far the result looks good, but it might be worthy to do some global edit things in reference manager before the export, like making author and journal names (abbreviations!) consistent before the export. As far as I know zotero lacks such functionality.

Nice to have:
The original reference manager ID appears under "Extra:" in zotero if you change
"ID - " to "M1 - "
This way I could import also Availability (AV - ) and Reprint (RP - ) into "Extra:".

I just face the following problem: Autor initials got imported as Mister M.W. for example. No Space between M. and W. In the bibliography I get

Mister M instead of Mister MW. Adding a space solves the problem.

Is there anything I have missed? Or do I really have to add spaces manually to all references that I want to cite in a paper????

I tried several styles. Always the same problem...

@rintze: Thanks, that's good news! Maybe I stay with 2.0 for my current project. But I'm really happy that I do not have to edit all 3600 references ;-)

@epilepsy: you can use saved searches for this: http://www.zotero.org/support/finding_and_sorting#saving_searches

For the notes, use "Note" "contains" " " (use a space).
For the images, use "File Attachment Type" "is" "Image".

Select one of the saved searches in the left column of the Zotero pane, and select all matching items in the middle column using a shortcut key (ctrl-A on Windows). Right-click one of the selected items, and select "Delete Selected items from Library..." (only notes or images should be highlighted; be careful not to accidentally delete the parent items).

And, most importantly, I'd recommend you back up first: http://www.zotero.org/support/zotero_data#how_do_i_back_up_my_zotero_library

sorry for the late reply:

your suggestion works very nicely for the notes, rintze, only the image search doesn't yield a result.

maba, you're right as well. the "content" of N1 is important and only put into the notes - and not where it properly belongs, for example "LA" isn't put into zotero's "language", the ISSN is only in the notes as well, etc.
please tell me there's a way around it. ;-)

i've tried the RTF export of both RM9 and RM12, but the format of N1 looks alike. i could send you a text sample as well of course to show you everything in detail and if you're willing to help me!

it says attachment and it's the icon with a chain on the bottom. i've already tried all the other file imports and neither seems to work for it. regular pdfs and screenshots are perfectly recognised.
another thing, it imports a folder with the imported file. but the publications are imported twice. so even if i remove the "imported" folder i have to look for them in the main library folder and remove them manually ... very annoying. spent 2 hrs today doing so with all my test trials. :(

@ icon: it's blue.

the 'double items' seemingly were some left-overs. my mistake.
you're right, removing a collection isn't deleting the items in it - and there's no option for doing that. but then, one can always go around this by simply selecting all items inside of the collection and bulk-deleting them. ;)

thanks a lot again!