If you have, in fact, purchased a legal copy of SS2008, then you have been provided with a unique serial key—either on-line or in the box.
All you have to do is enter it accurately into that one box in your forum profile. If that key is found, upon checking, to be a valid key that has not been previously registered by someone else, then you are granted access to the privileges of a legitimate user.
I will admit after reading many semi-literate posts of juveniles—of whatever age—that the simple task described above may beyond your capability, nevertheless it is necessary because pirated copies are readily available from many sources.
The only real incentive to pay for the game is the offer of access to multi-play, mission downloads, and technical help to registered users. And God knows you really need that last item. I sometimes wonder if all those bugs are part of an anti-piracy measure.