Mopizappuu

This happened to me as well. I tried all the usual Steam fixes but nothing worked. Eventually I gave up, uninstalled Steam, deleted its folder in ~/Library/Application Support and started over. The game launches fine now. This is not a great fix - you will have to download all your games again - but it seems to work in a pinch. Edit - here's a link to the Steam page describing the uninstall on Mac, you just drag the app to the trash and trash the Steam folder in your local user's Application Support folder: Removing and Reinstalling Steam on a Mac