Add items by identifier is a wonderful tool--when it works
I've had a surprising number of instances when the add-by-identifier feature returns the following: "Zotero could not find a record for the specified identifier. Please verify the identifier and try again." Yet, ZoteroBib finds it immediately. On rare occasions, I have closed Zotero and tried again later. On the second attempt, the tool found the item, but not usually. Different algorithms, I suppose, but it is puzzling and time-consuming.
This being said, can you give a few specific examples? "add by identifier" includes several different cases, it would be good to determine which one(s) are causing your problems and hopefully reproduce and analyze them.
Anyway, specific examples are still essential when it comes to analyzing metadata import issues. Can you post a couple of those and describe what's going wrong with each of them?
That said, the browser icon used on books found in library catalogs is indeed one of the best way of adding books, too.
I'm guessing you can't access that page unless you have an MCPL library card.
It might be helpful to you that the Library Catalog info pulled in is: mcpl.aspendiscovery.org
Is this what you're looking for?