If the network is MAC-locked (i.e. is only accepting traffic from known MAC addresses), you're probably out of luck - it's not going to listen to your machine, anyway (and it's tough to change MAC addresses in Mac OS X)

Having said that, have you tried tcpdump? It's built into Mac OS X and should show you any traffic passing across your NIC:

> sudo tcpdump -i en1

An alternative, wireless scanner is MacStumbler

It'll show you the wireless networks in range.