Just regedit the key in then?
Also, you keep saying "mount and blade" which reffers to the original game. That does not work with NW. Only warband.
But perhaps you knew that already.
I accidentally entered a wrong character when trying to join a server for the first time, Now I can’t join a server! I just get a “Invalid CD Key” error message.
1.Click Start > Run
3.A new window will open, browse to HKEY_CURRENT_USER > Software > MountAndBladeWarbandKeys
4.Search the entry, serial_key_nw, make sure that the value is the same as your CD key. Also make sure that the key is separated correctly. Double click the entry and modify it until it is correct. Watch out for invisible spaces in the CD key.
5.If you are done with that, close the Registry Window and start the game, connect to the server and have fun playing!