It's either in the cd case or with the manual. Apparently different releases of the game have it in different locations (mine is the case, but apparently for most people it's in the manual).
If you bought the digital version, open impulse and right click on the game and select the "view registration" option, and this will tell you your cd-key.
If you bought the game used... sorry to say this, but there's a huge problem with people returning the game but keeping the cd-key, effectively leaving you out in the cold.
