Activating a Riot key is really easy, simply follow these steps:
1/ Log into League of Legend user or go here to register.
2/ Move to Store tab.
3/ Select "Purchase RP" in upper-left corner.
4/ Choose the "Prepaid Cards" option and put your code in.
5/ Click on "Submit" and your Riot Points will be added to your LoL profile.
6/ Return to game client site.
The key is not working
If you have activated the key according to the guide but the code appears not to work, please check what type of error message you receive and depending on it follow the appropriate instructions: