One of the new features of Mountain Lion is "Gatekeeper" to protect the Mac from malware.
The default setting of Gatekeeper (find it in the preferences => Security):
Apps from the App Store and apps that are signed with an Apple Developer ID.
If Photo Mechanic has been present on your Mac before upgrading to Mountain Lion then it will open up without changing Gatekeeper preferences.
As Photo Mechanic is (currently) not signed with an Apple Developer ID it will not start if Mountain Lion is already installed on your Mac before you add Photo Mechanic to your Mac.
Apps that are not signed will refuse to start if you just double click the app. (screenshot 1)
Instead of changing the Gatekeeper prefs simply do a right-click (or ctrl-click) on the Photo Mechanic 5 application and select "Open".
You are presented a warning. If you click "Open" in this warning (screenshot 2) the Photo Mechanic 5 application will open and your choice is remembered for the current version of this app.
After an update you have to right-click (or ctrl-click) again.
Hope this helps
[attachment deleted by admin]