Click on the orange store icon on the top-right bar.
Then click "Codes" on the left-hand side, enter your key code and click "Submit".
Tristana Riot Girl Skin Key Giveaway
If you haven't already, get the classic Tristana Riot Girl skin with this Alienware Arena key. There's no reason why any League of Legends player should miss out on this skin, which will also automatically unlock Tristana upon redemption.
This is a must have for new players or for those that may have missed this free skin and champion.