i guess you may have to go put manually your serial key in registry
1. Start up registry editor, by going into the start menu, clicking run, then typing "regedit" (without quotes), and press enter.
2. Navigate through the following folders:
HKEY_CURRENT_USER > Software > MountAndBladeWarbandKeys
3. Enter the serial key in "serial_key" on right side of the registry editor by double click on it.