Roger,
It appears that Photoshop CS6 does not preserve our IPTC fields when saving from a RAW file. So you'll need to have PM read XMP before IPTC for JPEG, TIFF, PSD, and DNG photos. I would recommend that you also read embedded XMP before IPTC on TIFF-base RAW files as well. All of this is controlled in the IPTC/XMP section of the Photo Mechanic Preferences dialog.
HTH,
-Kirk