1. To validate your code, visit the official Rockstar page : http://socialclub.rockstargames.com

2. At the top right of the screen, create an account or login with your Rockstar account :

3. Once loggued, click on the settings icon at the right side of your nickname :

4. In the listing, select "Game Downloads" :

5. Choose "Use Rockstar code" :

6. Validate the code you received :

7. Confirm your purchase. If the Rockstar launcher doesn't work, change your Windows Username avoiding accents and dash.Check this link for more information http://www.tomshardware.com/news/grand-theft-auto-rockstar-installation,28930.html