Go to this link: RockstarGames.com
Use your existing Rockstar Games Social Club account to sign in. (Don’t worry If you do not have an existing Social Club account, follow the directions to create one and then retry the link).
Click on the "USE ROCKSTAR CODE" and it will be prompted for you to enter the unique Rockstar Activation Code. Then click on the ACTIVATE button.
Click the CONFIRM button for the site to confirm that you are activating for example GTA V PC.
To start downloading, click on DOWNLOAD FOR WINDOWS to download for example the GTA V installer on your PC and wait the downloading process.
To complete the installation process just follow the instructions in the installer.