I downloaded SourceTree and tried to install it on my Mac. I tried to open the application, and I got the following error:
"Sourcetree" can't be opened because Apple cannot check it for malicious software. This software needs to be updated. Contact the developer for more information.
My example shows Sourcetree, but this solution will work for any program downloaded from the internet onto your Mac.
Steps to open the application:
Open your System Preferences and click on "Security & Privacy
Click the lock to make changes and then click "Open Anyway".
You should now be allowed to open the application.