To redeem your RuneScape gift code on your computer simply follow the steps below:

Visit the RuneScape card activation page and click on Activate Card , https://www.runescape.com/store_locator
Sign in to your RuneScape account, or create a new RuneScape account for free.
Enter the RuneScape gift card code that you have received from us by email.
Redeem your code and start playing!