It could be on the instructions booklet or like when I took my game back 3 times because I could not find the key and If I had taken out the CD I would have found it. It was taped to the inside of the case under the CD. Good luck to you and I hope you find it.
If the product key is not where it is supposed to be in the manual, then contact the seller to ask for an exchange because your copy of the game is "defective". If you purchased it online, then most likely you will need to pay return shipping.