Try to manually add it?
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!