Since OS X Mountain Lion, the Mac add a new feature called Gatekeeper 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”.
If you sure the App you trying to open is safe, there are 2 ways to fix it.
1. Temporarily Get Around “App Can’t Be Opened” Gatekeeper Alert Message
This is probably the best option for most users, since it maintains some security:
Right-click (or control-click) the application in question and choose “Open”
Click the “Open” button at the next dialog warning to launch the app anyway
You can do this with any third party app that gives you this warning dialog and open it anyway.
2. Disable GateKeeper’s Unidentified App Developer Prevention Completely
This is generally best for advanced users who know what apps to trust and not to trust:
Launch System Preferences from the Apple menu
Choose “Security & Privacy” and then click the “General” tab, followed by clicking the Lock Icon in the bottom left corner to unlock the settings
Look for “Allow applications downloaded from:” and choose “Anywhere”
Accept the security warning and allow
You can now launch any app from any location or developer