You will have to log into Live Chess first :)
http://www.chess.com/livechess/myhome
Click on the orange button that says "Play Live Chess". This will log you into Live Chess.
Look at the Friends tab, this is where you will find the list of Friends that are logged into Live Chess right now.
Click on their name and you can Challenge them to a game. This will open up a window where you can choose the game options.
Let me know if you have any questions :)
All I can figure out how to do is challenge to online chess, but that's not what I'm trying to do. We are both online, we just can't figure out where we're supposed to go to do it. Thanks