by Venexis » January 13th, 2013, 12:49 pm
It might also mean that you're playing an unregistered copy of the game. Did you buy it from the official site?
If I remember correctly, there's a bit of a delay between the time of purchase and actually being able to log in using your account. Maybe give it a few hours and try again.
Finally, if that doesn't work, contacting the Mojang staff would probably be your best bet.

10/10, thanks FrozenFire 
Or add me, at Venexis#9902.