Hello! I also have a Mac. When the ZIP file's downloaded, all I do is double-click on it to extract Autonauts (Archive Utility automatically does this). I'm guessing that doesn't work for you? Hmm. I'll do some Googling and get back to you if I find anything that might help you :)
Yeah, I should've guessed from the title of your original post :)
In the absence of knowing what else to suggest (free zip tools from the AppStore?), I've recompressed and reuploaded the macOS Version 8, so download that and give it a try and let me know how you get on.
