In order to redeem a code on Steam, please follow the instructions below:
1) Open Steam window.
2) Go to the bottom left of the window and click "Add a Game"
3) Click "Activate a Product on Steam"
4) Click "Next" then "I agree"
5) Input the code in the "Product Code" Field and proceed.
This should redeem the code on your account and allow you to install it for use.