Click on the apple in the top left-hand corner of the menu bar and select System Preferences...
Click on Security & Privacy.
At the bottom of the window you should see "Autonauts" was blocked from opening because it is not from an identified developer.
To the right of that there's an option 'Open Anyway' - click that.
You should get another warning: "Autonauts is from an unidentified developer. Are you sure you want to open it?"
Click the Open option to run Autonauts.
You won't get the warning again unless you download an updated version.
Hope that helps. Let us know if you have any further questions.