Hi Ben,
The Library folder in the path is hidden by default. To access the Library folder, go to the Finder's Go menu. Hold down the Option key and now you will be able to open the Library folder.
Resetting Photo Mechanic's preferences will clear the Favorites panel entries and reset the IPTC Metadata Template, including any field popup list. All of the IPTC metadata applied to your images will still be present.
Best regards,
--Bob