Since the release of Mac OS X Mountain Lion applications from unidentified developers or sources are  prevented from being launched by default. This message will be shown in OS X 10.8 or later when you try to launch a Mac app that did not come from  the Mac App Store or from a verified source:

[App name] can’t be opened because it is from an unidentified developer.

The warning message looks like this:

Aerialist Gatekeeper Error

This issue is due to a new feature that was added in OS X Mountain Lion called Gatekeeper which keeps things more locked down. A range of temporary and workaround solutions can be found on this site which allow you to resolve this issue.