Telltale found a server issue that existed only in the build that was intended to be uploaded to the Telltale website. They've been trying to fix the issue all day, but it doesn't appear that they'll be able to fix it tonight.
Thankfully, everyone who pre-ordered the game from Telltale's site will be sent Steam codes to their e-mail addresses so they can play the game via Steam (and get the extras like Steam Achievements that go with it).
So, just be patient. They've been sending out keys for the past hour. You'll get yours soon. I'm in the same boat in that I just pre-ordered today. We'll be the last people to get the codes in our e-mail inboxes, but they'll be coming.