In AirPort Utility 6.1, click on the AirPort Exteme icon, then look for Wireless Clients. Click on the listing of each device just to the right of Wireless Clients and another box will open to display the following for each client:
Unfortunately, there is no convenient "list" of all conneced devices. You have get details on each one individually.
I don't know how to find an Ethernet client if you need that info. I think Apple left this out.
Heres one that I use. http://www.iwaxx.com/debookee/ When my network runs slow I pull it up just to check out whats going on to make sure I don't have any unwanted guests in my wireless network. Make sure you do a back up before installing.