You might want to download and install the much more useful AirPort Utility 5.6 for Mac OS X Lion
Both AirPort Utility 5.6 and 6.x can be kept on your Mac.
Open AirPort Utility 5.6, select the Time Capsule and click on Manual Setup
Click the Advanced icon
Click Logs and Statistics
Wireless Clients and DHCP Clients (which include both wireless and wired clients) will be displayed
This assumes that the Time Capsule is acting as the main router for your network.
If you only see a tab for Wireless Clients, the Time Capsule is in Bridge Mode and you will need to look for the information about wired clients on the device that is handling the routing services for your network.