First, you will need to have the Oculus Rift desktop application installed.
Once installed, you will want to launch the Oculus Rift application and log in to your Oculus Rift account.
You can follow the pictorial tutorial below to see how to go about completing the activation and installation process:
Open the Oculus Rift desktop application and log in.
Click the gear icon in the top right, then click Settings.
In your Account settings, click Redeem Code.
Enter your activation key, then click Redeem.
After successfully redeeming the key, you’ll have the option to “Install Now” or “Install Later”. If you install now, your game will begin installing in your library. You’ll have the option to “Install Now” or “Install Later”
If you install later, you can install the game by clicking the Library tab and then clicking “Not Installed”. Click your game to begin downloading it. You can install the game by clicking the Library tab and then clicking "Not Installed
Once the game is installed, click the game and it will finish installing and launch.