Since OS X Mountain Lion, the Mac defaults to preventing applications from unidentified developers or sources from being launched. You’ll discover the message in OS X 10.8 when you try to launch a Mac app that didn’t come from a verified source or from the Mac App Store, and you’ll get an alert dialog that says “[App name] can’t be opened because it is from an unidentified developer”.

This video shows how you can fix this error.

“app can’t be opened because it is from an unidentified developer”