Follow the steps here:
https://www.zotero.org/support/word_processor_plugin_troubleshooting#zotero_toolbar_doesn_t_appear

If you still have problems, please describe exactly what you have done and what is happening.

What version of Zotero are you running?

It's still happening. I go into Zotero ---- tools--- ad-ons and nothing loads. Just that spinning wheel forever.

HAH. Got it to work by installing this: https://github.com/zotero/zotero-word-for-mac-integration/raw/master/install/Zotero.dotm

I am using 5.0 version, followed these step as mention in link
Zotero toolbar doesn't appear

If the Zotero tab or toolbar (or Zotero scripts menu for Word 2008/2011 for Mac) does not appear, follow these steps:

Close Word or LibreOffice.
In Zotero, go to Tools → Add-ons → Extensions.
If “Zotero Word for Mac/Windows Integration” or “Zotero LibreOffice Integration” are disabled, click the “Enable” button to re-enable the extension.
If the “Enable” button for “Zotero Word for Mac/Windows Integration” is grayed out, and you are unable to click it, click the “Remove” button and restart Zotero.
If you don't see “Zotero Word for Mac/Windows Integration” or “Zotero LibreOffice Integration” in the Extensions window at all, then you should re-download and re-install Zotero. It is not necessary to uninstall Zotero before re-installing; simply install Zotero over the top of the existing installation. (Note: Re-installing Zotero rarely fixes problems and should not be used as a general troubleshooting step. This is one of the few instances where re-installation is helpful.)
In the Cite pane of Zotero preferences, open the “Word Processors” tab.
Click “Reinstall LibreOffice/Microsoft Word Add-in” and then restart your word processor.

But not still same problem, word 365 not showing tab of zotero.

Few month back i had used same version with word 2010 and it went well. but now i am using Word 365 its not working

followed all steps as mentioned above

Zotero Word for Windows Integration this option and its remain enable

You can find here the procedure :
https://forums.zotero.org/discussion/73354/how-to-use-zotero-plug-with-word-365#latest

Thank you it also worked for me. just follow this link
https://support.office.com/en-us/article/load-or-unload-a-template-or-add-in-program-2479fe53-f849-4394-88bb-2a6e2a39479d

when you try to select the add ins for word, go to zotero folder c drive. For my computer following was the location
C:\Program Files (x86)\Zotero\extensions\zoteroWinWordIntegration@zotero.org\install

in the final install folder select the file named 'Zotero.dotm'

check your word processor it should now contain the zotero tab

@carlacruzrv: See bwiernik's original response above. The same applies in your case.

@carolyn wolf-gould
July 6, 2018
HAH. Got it to work by installing this: https://github.com/zotero/zotero-word-for-mac-integration/raw/master/install/Zotero.dotm

Worked on Windows 10 with Word 365. Thanks!