If you bought the game retail, it should be on a sticker in the package; it's generally on the back of the manual.
If you bought it online through Impulse, you can go to https://www.stardock.com/support , enter your e-mail address to have your product info sent to you.
If neither of those help, you can drop an email to support@stardock.com, and we can try to get you up and running.