Roger,
In the Files section of the Photo Mechanic Preferences dialog where it says "Add tag, color class, and rating to IPTC/XMP:" make sure the popup menu says "For all photos". You did not have the setting set as I had asked you earlier. Perhaps I wasn't clear.
Then update any one of those settings (toggle tag twice will suffice) on a photo and then try modifying that photo with Photoshop. The problem should be solved.
-Kirk