The Airport Express manual means that the antenna came with *the computer*. No *additional* antenna need be installed.
The antenna was in the packaging that the G5 came in. You bought an incomplete computer. Your computer’s previous owner didn’t fully assemble the machine.
It’d be like buying a Dell laptop with no battery and then whining that the thing wouldn’t work without the power adapter plugged in - you bought an incomplete system.
Well to make matters worse, I didn’t really even buy it. It’s payment in lieu of another debt, so It’s kinda free.
Still, the dudes at the Apple store put the thing in and I even asked them about the antenna, they said it doesn’t need it.
I will admit that I can’t really gripe too much about it since it’s “supposed” to have one, but nary an “expert” pointed that out until you. Can’t wait until I buy an external HDD, I imagine that may take 3 or 4 trips to the Apple store to get it working right.