Ever seen ‘”.app” can’t be opened because it was not downloaded from the Mac App Store’, gone into system prefs, clicked the padlock, entered your password and temporarily switched the ‘Allow apps downloaded from:’ to ‘Anywhere’ in order to open your new app?
This feature is called Gatekeeper and prevents any app made by a developer that isn’t certified by Apple from running on your system.
It’s best to leave it on ‘Mac App Store’ to protect yourself from malicious programs trying to run especially as you can run your new app with three clicks.
Simply ⇧ Click (control + click) on the app icon and select ‘Open’.
You’ll then get this dialogue box. Select ‘Open’ and your new app will run.