If you have just purchased the game as a retail copy the key should be located inside the case on a reference sheet. If you have purchased the game as a digital download there should have been an email sent to you with the key.
Keys for this game can only be activated one time. After the initial activation, the key can never be used again. So, this problem is only caused by one of two things:
If you previously activated the key and are being asked to enter it again because you are reinstalling the game, you are simply at the wrong login screen. Please follow the instructions in the following FAQ for assistance with getting to the correct login screen, where you can enter your Uplay account name and password: