Yeah, the envelope would be a good option, if no documentation whatsoever was included.

On mine, the CD key is on the (very thin) booklet inside the front of the CD case, on the foot of the very last page....I got mine online a while back....

Fingers crossed that it is on that envelope - if not, then heed the advice of Cynewulf. Not saying that it is not "ridgey didge", but the seller may be able to help you to locate the CD key....