Can't register the game

Short story: There is nothing wrong with your key, but Paradox has not (and will not) receive a usable copy of it from Taleworlds and can thus not use it to acknowledge your ownership of the game. All M&B subforums have been opened to accomodate those who can not register their game on the forums and all you are missing out on is the icon.

Long story: There are pages of threads I could refer you to, but basically they say what I said above.