Open the Preferences dialog and select the GPS page. Check the "Disable Reverse Geocoding subsystem" checkbox. Is it checked or unchecked?
It needs to be unchecked for Reverse Geocoding to be able to be used.
If that subsystem failed to start up in the past, a dialog would have appeared allowing you to disable it and if you indicated that you wanted to disable it, PM set that checkbox for you. Uncheck it and restart PM. If it fails to start up again, please post your log file. To post your Photo Mechanic log. In Photo Mechanic go to the Help menu and choose "Reveal Support Data..." and moments later a Finder window will come forward with the zipped log file selected. Use the 'Attachments and other options' link when you're composing your reply to this message and there you'll be able to upload your zipped PM log file.
Thanks,
-Kirk