Click the "Chess" tab. Then there should be a bunch of other tabs above the chess window, on the right side of the screen. Click "Challenge". There should be a few challenges other people have already started; you can push "Click to Join" on the right side of the window if you want to play those people. Alternatively, you can push "Create Quickmatch", then select your time limit and variant (and add a message to the challenge if you wish), then push "Challenge". That will add your challenge to the list. You'll get a message when somebody joins your game.
Also, you can go back to those tabs at the upper right of the chess window and click "Leaderboard". There should be a list of players by rating. If you click the "[c]" next to any of their names, it will send a challenge to that player (and only to that player).
Hope this helps.
EDIT: also, there are several board themes that don't display the chess tab. I'm not sure which do, but I know that the default theme does, so switch to that if you want to play chess.
