Make sure you downloaded the Mac OS version of the game! (Some people have downloaded the Windows dev-build version by accident) It should be a .zip file called "BBE_0.2.1_Mac.zip" which you need to extract. Inside the extracted folder is the BBE app it should be as simple as running it at that point.
Let me know if you have any issues! (I don't actually own a Mac device so I can't verify this myself.)