Hi Andrew,
Make sure you are running Photo Mechanic build 7765. Now go to Photo Mechanic's Accessibility Preference page. Click the Metadata (IPTC) Info button in the Customize section. Scroll down to the bottom section of the list. The Data Mining option is above the Getty Images section. Check both boxes and then click the OK button. You can also click the Restore Defaults button to add the Data Mining field and any other missing fields.
You will need to do the same thing for the Metadata Template. Just click the Metadata (IPTC) Template button and then click the Restore Defaults button.
Best regards,
--Bob