Version 6.x of Apple’s AirPort Utility displays all your wireless client’s IP addresses, but it doesn’t show you wired devices. The old version 5.6 did, but without a hack it no longer runs on Mountain Lion.
There is however a simple way to display all clients using the command line tool arp. Open up a Terminal Window (Applications – Utilities – Terminal) and type the following:
arp -a ? (10.0.1.1) at 88:1f:a1:2c:87:37 on en1 ifscope [ethernet] ? (10.0.1.2) at bc:52:b7:1a:f2:9 on en1 ifscope [ethernet] ? (10.0.1.6) at 74:e1:b6:c0:31:f1 on en1 ifscope [ethernet] ? (10.0.1.18) at bc:67:78:55:37:96 on en1 ifscope [ethernet] ? (10.0.1.24) at 4:c:ce:dc:94:4e on en1 ifscope [ethernet] ? (10.0.1.25) at 88:1f:a1:2c:87:37 on en1 ifscope [ethernet] ? (10.0.1.29) at 0:13:77:d4:da:d0 on en1 ifscope [ethernet] ? (10.0.1.255) at ff:ff:ff:ff:ff:ff on en1 ifscope [ethernet]
It’s not pretty, but it gives you a good idea of what’s attached, and which IP addresses to try. You can also compare your wireless devices to this list and see which one is the “odd one out”.
To display your wireless device IPs, open AirPort Utility, then option-double-click onto your AirPort model/icon and a window opens up. Each device has “friendly name” and a small triangle that will display its IP and MAC address:
arp -a only lists devices that computer or device has seen since it started.
If you are running it from a mac on a switched network, it will only see what has connected to that mac. You would need to be able to run arp from the time capsule itself to see what has connected to it.