Hm... I haven't played any demo's off steam, so I may be out of my element, but I'm guessing its downloading an update or something, or perhaps steam is off for whatever reason. Try going to your library, right-click on it, and choose "properties". Then go to the "local files" tab and click "verify integrity of game cache".
If that doesn't work try restarting Steam and/or redownload the demo.
Other than that not much else I can suggest.
And I feel I must say this, my suggestions are for the PC, and since I know little to nothing about Macs, my advice may not work. But its worth a shot.