Hello, thank you for buying my game, I appreciate it!:)
Since you're playing on Mac, unfortunately you'll have to do a little extra work to get the game to work on your device. 

Follow the instructions at this link: https://help.yoyogames.com/hc/en-us/articles/216753558-Mac-app-damaged-when-downloaded-from-the-internet

Sorry for the inconvenience, this is just something us indie developers have to deal with if we're distributing our OSX games anywhere other than the Mac App store.