Get your activation code, via either the website or if you otherwise stored it, download Steam (or presuming you already have,) open Steam. At the very top left press 'Games', click 'Activate a product' and put in your activation code. And that's it. Unfortunately, you'll have to do this with Warband as well, since this is an extension of warband and it may not recognize you already have it. Additionally, you will need to (re)install it from Steam in order for the patches to be downloaded.
I realize this is extraneous effort on your part, but it'll at least give you access. Obviously this is to the fault of the developers or Taleworlds, but at least this post should make him aware, and my post should give you a temporary alternative.