OS X El Capitan: Open an app from an unidentified developer
If you try to open an app that is not registered with Apple by an identified developer you get a warning dialog. This doesn’t necessarily mean that something’s wrong with the app. For example, some apps were written before developer ID registration began. However, the app has not been reviewed, and OS X can’t check whether the app has been modified or broken since it was released.
A common way to distribute malware is to take an innocent app and insert harmful code into it, and then redistribute the infected app.
The safest approach is to look for a later version of the app from the Mac App Store or look for an alternative app.
To override your security settings and open the app anyway:
In the Finder, locate the app you want to open.
Don’t use Launchpad to do this. Launchpad doesn’t allow you to access the shortcut menu.
Press the Control key and click the app icon, 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 registered app.
To open this pane, choose Apple menu > System Preferences, then click Security & Privacy.
To change your security settings, see Protect your Mac from malware.