Install by downloading the latest version

Fetch PMCID/PMID for items with a DOI. Select items, right-click, Fetch PMCID

Or, if you always want them:

• Open the Zotero hidden preferences
• Create a new key with name extensions.zotero.pmcid.auto, type boolean, and set it to true
• Add or modify items and PMCIDs/PMIDs magically appear

My time is extremely limited for a number of very great reasons (you shall have to trust me on this). Because of this, I cannot accept bug reports or support requests on anything but the latest version. If you submit an issue report, please include the version that you are on. By the time I get to your issue, the latest version might have bumped up already, and you will have to upgrade (you might have auto-upgraded already however) and re-verify that your issue still exists. Apologies for the inconvenience, but such are the breaks.