I'm sorry for the trouble with that. The game isn't officially approved by Apple, it's a third party app. You can still download it for Mac if you want to, but getting it to work can be annoying because Apple tries to stop people from using third party apps. But either way, the DLC files are exactly the same on Itch and Steam. So you can download it from Steam, look in the local files for the DLC rpa files, and then move those rpa files into the Itch Mac version.