1. If you have not yet downloaded the uPlay client, make sure you have done so.
2. Open up the uPlay Client and log into your account. From there, click the "Games" button at the top. You will have three options available. Click the "ACTIVATE PRODUCT" button and enter your Key.
3. Verify you are activating the correct title and redeem your game by clicking the "ACTIVATE PRODUCT" button. Once this is done it will automatically add the game to your library.
4. That's it! Once you've located your game you may go forth and play! If you're unsure if your key activated, go to your Account Details to check. For all other issues, please contact Ubisoft Support here.