One way is through Airport Utility. Click on your Airport device on the left, go to the Base Station menu, click Logs and Statistics, and go to the Wireless Clients tab. You'll see clients listed by MAC address. If you're running DHCP on the Airport you'll also have a DHCP tab showing IP addresses of clients.
I found this:
"You can choose to allow SNMP on the wide area network (WAN) port by selecting the "Allow SNMP over WAN checkbox." Allowing SNMP over the WAN port lets you mange your base station remotely."
Remotely means over the internet? I didn't get it.