If you've purchased the game, you can find the activation key or product key on the back of the game case, on the receipt, or in the email confirmation if you bought it digitally.
That being said, I can offer you some general information about NFS Shift 2 and its activation process.
Need for Speed: Shift 2 - Unleashed is a racing video game developed by Slightly Mad Studios and published by Electronic Arts (EA). To play the game, you will need a valid product key or activation code.