Did you get it Online from Stardock? I'll be typing, editing this as you reply, for the ones I know too.

EDITED: OOPs rereading your first post.. realized you couldn't have got it from Direct to drive.

Check the manual or game case for the Key/serial to play online, registration I am not sure though.

You can add Serials to Stardock Central, but I am not sure how it works. First Contact Stardock by email and see what they say to do. Explain things as clearly as possible so that the process goes by fast. By the way next time, get the game from Stardock Central. Much easier process, with Automatic Updates.

In the Register Products window of Stardock central, is a location to enter Serials.

Damn, I just erased the stuff I typed to get it from Stardock, Doh.

I'll type it again, I knew I should have not been rushing...

Start Stardock Central.
MAKE SURE IT IS THE CORRECT EMAIL ADDRESS, THIS IS VERY IMPORTANT!
Select Tools in teh Menu (top of Window), then "Lookup Serial Numbers"
OR
Select Tools in the Menu (top of Window), then "Register Products"
Click on "Lookup Serial Numbers", then it should send ALL your serial numbers and registration info to that email address. Again, make sure it is the correct email address, as if its wrong, your information could be sent to a anonymous person.