I had this problem a while ago, BC (Before Cable)
This from the book
Connecting to the Internet Via the AirPort Network
If you are not connected, open Internet
Connect, located in the Applications folder, click AirPort in the toolbar, and click Connect.
Try this, if you haven't already, and let us know what results you get
Hope it helps
How do I configure my wired G4 and my wireless PBG4 so that the wired G4 can initiate dialup to my Airport Base Station (graphite or snow)?
Right now, if the PBG4 connects to the LAN and initiates the dialup, the wired G4 can join the LAN and share the internet. But how do I configure the wireless and the wired so that the wired can initiate the dialup?