One note to add. Please check the Futuremark Support FAQ under
http://www.futuremark.com/support/3dmarkvantage/ - there it is
said that you sometimes have to enter your serial twice. It's a
known problem, so nothing to worry about (it's not our fault).

So, first time when u enter the key it might be not accepted,
just re-enter it again and it will be accepted.