There is a tiny bug in the demo that we are fixing right now. When you launch the game you'll be prompted to log in. If you haven't bought it yet, you'll get an error message telling you to email support. Don't! All you need to do is click the "take me to the demo" button and you can proceed as normal.
If you HAVE bought the game and are getting an error about "multiple serials" when you try to log in, just click the "take me to the demo" button and activate using your serial on the Serial Unlock tab. You can find the serial number on your My Telltale page or in your email confirmation.
I'll take this thread down when the problems have been fixed. Fixed! Actually like eight hours ago, but I forgot to take the thread down. :p