monetize a mac game?

The same way you monitize any other game.

The only real difference for MacOS development is who your target audience is, and how large the audience is. It isn't too difficult to extend it to all flavors of *nix, so you can greatly increase the audience size with less work than porting a Windows app.

You certainly can stick ads in the games, and many people do.

Ads are not the only way to monitize a product; traditional sales, shareware, trial versions, "lite" and "pro" editions, SaaS, and many other methods exist.

The easiest way that come to mind is the App Store...

When you say a "Mac game", are you talking about a game that would be played on Mac computers? Because those games are typically monetized through the premium model (you pay $1 to buy the game) on distribution platforms such as the Mac App Store and Steam

If you meant "iPhone game", then you probably want to stick to a freemium model. "Freemium" means having your game be free to download, but you get users to spend money within the game. This can be for more levels, in-game items, or unlocking more features.