The AirPort card is located next to the graphics card.
If your computer has a FireWire 800 port between the FireWire 400 Ports and the Ethernet Ports then your computer uses an AirPort Extreme 802.11g Card (See Related Parts). If your computer has no firewire 800 port, then your computer uses an 802.11b AirPort PCMCIA Card (Also See Related Parts). Removal of both cards is pretty much the same.
First disconnect the antenna from the card, it may be helpful to use a spudger or flathead screwdriver.
Now, grasp the plastic tab on the back of the card and pull it out of the computer.