You will need to lift the keyboard off your iBook to gain access to the Airport Extreme Card:
Make sure the keyboard locking screw, located in the small plastic tab to the left of the Num Lock key (see pic below), is not in the locked position. Your iBook comes with the keyboard unlocked, so unless you or someone else locked the keyboard, you can skip this step. To unlock the keyboard, turn the screw 1/2 turn.

Next: Release the keyboard by pulling down on the keyboard release tabs (located to the left of the F1 and F12 keys) (see pics below), then lift the top portion of the keyboard up slightly, and toward the display.

BEFORE:
   
AFTER:
 

We can now flip the keyboard over and lay it on the palm rests and trackpad (as shown below). This reveals the Airport Express Card!!!!