On the G5 dual 2.0 the AE card installs in a slot immediately inside the system on the left when the side cover is removed. It's very easy - there's a frame/guide the card slots into and is then pressed (firmly) onto the connector and the airport antenna lead pressed firmly into the connector on the back edge of the AE card - it all takes about 30 seconds.
The only thing to be cautious of is that the card is firmly seated and the antenna firmly attached.
The T-shaped antenna must, of course, be attached to the connector on the back of the G5, and if the G5 itself is between the antenna and the WAP, it will cause the signal level to drop. My WAP is located behind the G5 so the signal is good - if I turn it round, it drops from 4 bars to 2!