Don't worry man, I haven't had the game registered since a year ago, more or less, because I got it from Steam. As Grombeard says, it's a well known issue, so nobody would ever tell you anything about not having registered it. They opened the forums of M&B Warband completelly because of that.
The problem seems to be that TaleWorlds sent a huge bunch of keys (all of them supposed to be activated in Steam) in a strange format that Paradox could not interpret correctly in order to register them in their database in order to be used at Paradox Plaza. Fixing it manually takes much time for the staff and the only thing affected is that you won't see the game icon under your name nor in the My games section, but nothing else, so they won't be doing it anytime (I already asked for it last year and I didn't get it registered with the Steam key).
You can always buy a key from GamersGate (it's 75% off these days), if you want to register it here, and then you would have 2 valid keys to use anyway, so a friend of yours would be grateful to receive the second key as a gift, for example
I hope it helps!