Dioni,
If that person is an official chess.com Friend, his/her name will show up in your Friends tab IF AND ONLY IF they are actually in Live Chess when you are there. If you see the name under that tab, click Play and you will have a game with that Friend.
If the person is NOT an official chess.com Friend but is in Live Chess when you are, then look in the Players tab and find that person, then click Play to get a game.
You must BOTH be actually in Live Chess to challenge one another to a game of Live Chess.
I know how to challenge a friend to an online game, but how do I challenge a friend to live chess?