Oh no! Let me take a look and see what I can do. Thanks for the heads up.
Update 1: It looks like it is a Certificate signature problem. I will have to look into getting it fixed. I'm taking the Mac download down in the meantime, I apologize! Thanks again for letting me know.
Update 2: I'm really sorry to come with disappointing news, but after some investigation, it seems I need an Apple Developer account in order to sign the app for Mac OS. It costs $99 for a membership, so I don't plan on doing that at the moment. If you've already downloaded it for Mac, I believer there are ways of bypassing the certification check, but no worries if you feel uncomfortable doing that. Sorry again!