By default, OS X lets you run/install apps from trusted sources only. That's why you need the Mac App Store to install apps and that's why, when you are trying to install an app that is not “identified” (approved by Apple), you won't be able to.
This disabling comes from Gatekeeper, a system in OS X that tries to protect you from malware. If you have enabled Gatekeeper, you will not be permitted to open any unauthorized apps. You will always see an error message popping up saying, “New App can't be opened because it is from an unidentified developer”. This security check will affect all downloaded apps and the ones you are going to download later.
However, if you are sure that the app is malware-free and it will not harm your Mac, you can bypass Gatekeeper and open the app by following the steps below.
How to Open an App from Unidentified Developer in Mac OS X
Step #1. From the Finder, search the app you want to open.
Step #1. If the app is on your Mac desktop, directly move your cursor on it.
Step #2. Press Ctrl + click on the app to open it. Or, you can right click on the icon of the app to open the context menu.
Step #3. Click on “Open” from the context menu.
Step #4. Instead of the error message, you will see a popup menu. Click on “Open” and the app will launch.
The problem is that you will have to do this every time you want to open the app. Changing Gatekeeper settings in System Preferences → Security & Privacy can help you override the error message. To do this:
- Open System Preferences
- Click on Security & Privacy
- Select “Always” from the ‘Allow applications downloaded from: label.
This will prevent Gatekeeper from throwing up those warnings all the time. However, note that this is a risky affair. You need to be absolutely sure that what app you are running will not break your security/privacy.