Apple has a built-in way to protect you from opening up potentially malicious apps on your computer. This software, named Gatekeeper, won't stop you from opening apps installed from the App Store , but it might if they come from elsewhere. If you want to open/install an application from the internet on your system, here's how to do it.

1. The quickest way to open an application when you get an Unidentified Developer warning message (as seen below) is to Right Click and select Open.
   
    
2. An alternative method is to open your System Preferences  and go to Security & Privacy.
   
    
3. In the General tab, you'll see a section called Allow Apps Downloaded From. In order to change these settings, you'll have to click on the Lock Icon below and input your Username and Password. Once that's out of the way, select App Store and Identified Developers.
   
    
4. Return to the App that you were trying to open earlier and Reopen it.