When you first log into Challengermode, click on "League of Legends" in your left sidebar navigation.
You will then enter the League of Legends game page which will look like this.
Add a LoL Summoner
To add a League of Legends Summoner, simply click on the "Add Summoner" button which is displayed below the game title.
Type in your Summoner (the name of the LoL game account) and choose the region. Europe West (EUW) and Europe North & East (EUNE) are currently supported.
Once your summoner is found you can choose to verify it manually if you want to play in the daily ladder right away.
If not, you can add your Summoner anyway. (Your summoner will be verified automatically if you play a tournament or a challenge using a Tournament Code.)