Using iTunes with Airport

Sometimes when I open iTunes, there is a pull-down menu bar at the bottom of the screen that lets me choose between the computer and my airport, which plays from my computer onto my home stereo. BUT sometimes, this menu bar disappears. Even if I re-start iTunes or my whole computer, the menu bar is gone. I wish it was more consistently available. Does anyone have any clue on how to prevent this menu bar from doing a disappearing act?? I like to broadcast my playlist to my stereo for parties, housework, and Friday night unwinding!!!!