Resetting Photo Mechanic's Preference files will erase all of the entries in the Favorites pane and Open Recent list. If you need to restore your Favorites entries, then Take a screen shot of them before resetting PM's preference files.
If you have FTP connection settings or IPTC field lists that need to be preserved, then go to the Photo Mechanic Preferences dialog and click the Export button to save the current settings as a .PMX file. Call the file PM-old-prefs.PMX and save it wherever you like. Now quit Photo Mechanic. Use the Command-Tab shortcut when the Finder is running to show a list of all of the running application icons. If Photo Mechanic is not listed, then we can continue.
The Photo Mechanic preferences files are located in your user Library folder -> Preferences folder. Unfortunately this folder is hidden on the new Mac operating systems (Lion and Mountain Lion). We can get around that problem: with the Finder running click the Desktop so the Finder is active. Hold down the Option key and click the Go menu. Keep the Option key down so you can access the Library folder from the Go menu. When the Library folder opens, you can drag the Library folder icon from the top of the Finder window, to the left sidebar next to the Desktop entry. Make sure you see the separating line between two entries in the sidebar before letting go of the mouse. This will add the Library folder shortcut to the sidebar.
Now open the Preferences folder. Look for a folder called com.camerabits.PhotoMechanic. Rename this folder by adding -old to the name. If there are any com.camerabits.PhotoMechanic.plist files, then you can delete them or add "-old" to their names. There could be 3 of these plus 3 copies ending with Lockfile. Close the Finder window.
Start Photo Mechanic. It might complain about the registration. If it does and you do not have your registration info handy, then quit Photo Mechanic. Go back to the Library folder and then the Preferences folder. ( Library folder shortcut will be in the left sidebar of any Finder window ).
Photo Mechanic should have created a new com.camerabits.PhotoMechanic folder. Open the com.camerabits.PhotoMechanic-old folder and move the com.camerabits.PhotoMechanic.registration file to the new com.camerabits.PhotoMechanic folder.
Now Photo Mechanic should be reset so it looks as if it has just been installed for the first time. Try the same workflow to make sure Photo Mechanic is working correctly now. If not, then I recommend re-installing the Photo Mechanic application.
If Photo Mechanic is working correctly now, go to the Preferences and click the Import button. Locate the PM-old-prefs.PMX file from where you saved it earlier and Open it. When the list opens click the Import button to continue. Click the Yes button in the alert dialog to complete the Import. Photo Mechanic will need to quit to save the changes.