One more idea, I am not sure if it helps:

Are you absolutely positive you have entered the correct serial number? It is long and easy to get wrong when entering it. Perhaps one digit is wrong, and you have thus by mistake entered someone else's serial number and they have already registered?!

You can see what serial number you have entered in the "RealFlight Control Panel". It should be displayed above all those buttons.

Hope you get it working!


Krister