OS X Yosemite: Open an app by overriding Security & Privacy preferences
You can open an app that isn’t allowed to open by manually overriding the settings in Security & Privacy preferences.
In the Finder, locate the app you want to open.
Most apps can be found in the Applications folder.
Press the Control-key, then click the app, then choose Open from the shortcut menu.
The app is saved as an exception to your security settings, and you can open it in the future by double-clicking it, just as you can any authorized app.
For more information about protecting your Mac from harmful software, see Protect your Mac from malware.