@Zagnaros
Did you buy the game digitally from Stardock? If so login with the same account on Impulse and look under your games, It should be listed. If not contact the support email.
Did you buy a new retail game? Was it packaged and sealed? If so contact the support email.
Did you buy a used version of the game? Used versions are already registered and can't be re-registered. You might try contacting Stardock, maybe they will work something out with you (who knows).