I did a search for the xg-504, and THIS THREAD is number seven on Google's list of hits! Wow!
Anyway, USB wireless devices notoriously have issues with Macs, but it looks like that's not the issue here. So as Tuoder says, we need more information. Let's start out with what computer you're connecting it to, what OS version it's running, and how your AirPort Extreme is configured. There's a possibility that there's simply a setting (on the APX) that is preventing it from showing up.