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Airport Utility > (double click on your Airport) > Wireless Clients

That will show you the MAC address of everyone connected, anyway. You'll have to toggle over to the DHCP tab to see if it helps you figure out which user it is.
 

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I was going to say, .. I took a picture from my Time Capsule, but my Express shows the same option (it just didn't have any users connected to it, so I didn't take a picture of it). Glad you figured it out.
 
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