1.Go to: https://www.blizzard.com/
2.Log in with your username / password or create a new blizzard account if you don't have one).
3.Once logged in, click on the top left menu in Games & Codes and select: Add a Game Key
4.A new window will prompt you to enter the game key you have purchased on GK4A
5.Enter the code and click on REDEEM CODE button
8.Battle.net will now authenticate your code after a few seconds.
9.Go back to the Top menu again, Games & Codes and select the option: Download Clients
10.Click it to start the download of your desired game.If you require more specific help, or if you have any questions, don't hesitate to to Contact us by Chat or normal contact form.