Some users have reported issues with Game Center (a Mac based game platform) and Steam. Namely, that if under specific circumstances you attempt to launch PEG, a Game Center popup will appear and never allow for the game to actually launch.
This is a problem that comes with the integration of Steamworks for the game, which is mostly just for achievements. In other words, for attempting to launch the Steam toolkit, the Mac OS native Game Center also tries to startup, causing this problem.
If you are experiecing this issue, there is now an option to disable Steam entirely (just so you can play the game). While this won't allow the achievements to be unlocked on real time, the ones you completed will still be stored in your save file, so if you want for them to appear on your Steam profile, you just have to re-enable it and launch.
That said, to disable Steam for PEG all you have to do is right click the game on your Library > Properties > General > Set Launch Options...
And enter the following:
-disablePlatform
While minding the capitalization and initial hyphen, which should look like this:
Once that is done, the game can now launch without Steam functionality (though the Steam overlay will still be there), which should solve the issue. Where again, if you want to get the achievements you collected you'll only have to delete the previously added text and re-launch.
Hope that is of help for some users.