If you preordered it a couple of weeks ago, you'd have gotten a new email yesterday with your serial key. If you bought with the smae email as you use on the forums/Impulse, you won't even need it--just fire up Impulse and download the beta.
If you lost the email from yesterday as well, you can have the serial resent by entering your email in the recovery box at http://support.stardock.com.
