More specifically, for anyone who can't figure it out, the link to the steam group is on the first page of this thread. I'm assuming you've already joined it. After signing in, go to the discussions tab (default is overview). The discussion "Keys to get into dev app" is the one you want. Scroll down the comments until you find an unused key. Then go to Steam, look in the bottom left corner, and click "add a game" and then the option "Activate a product on Steam" When it prompts you for the code, enter the code from the steam group discussion. If it doesn't work try a different code. And voila, you're all set!
