To find out what stairs you can currently challenge someone in, first go to the Main Stairs Page, then click on "Show your stairs only" link. The ones in BOLD are ones you can make a challenge in. .
ouspensky: Select the "stairs" option on left hand side down from main page select type of game you wish to play eg backgammon then select fast/ standard etc. hope this helps, also note you can only enter 1 stairs. Good luck