The game will work perfectly fine if you use the itch.io app.
If you don't want to use the itch app, or if you downloaded it, you should be able to right click or control click on the game and select "Open".
You might have to click "OK" after it warns you “Game.app” is from an unidentified developer. Are you sure you want to open it?
The text on the menu didn't load for me at first, but it showed up after I pressed enter on the options menu (the bottom button), and I'm not sure if this is a cross-platform issue. Just letting people know in case they also run into that.
Temmie, if you could update the instructions on the main game page to help other people, that'd be appreciated!