Installing the Airport Card involves removing the outer case before actual installation.
Release the back handle of the PowerMac. Slowly pull out the cube by holding the end of the handle. Watch for any parts that may become entangled with the outer case.
Push back the handle after you have taken the core out of the case.
Place the cube upside down and remove the four T10 Torx screws on the front plate. Then remove the two screws on each side.
Remove the four pillars from each corner.
Pull outwards on the antenna cable.
Pull on the clear tab outwards to remove the existing AirPort card
Slide in the new AirPort card and connect the antenna cable