Which model TC do you have.. this is important??
A1xxx from the base please.
If it is not the latest one.. the easiest way is to NOT use the latest utillty.. simply load the old utility into your computer.
Here is one method.
How to load 5.6 into ML.
1. Download 5.6 for Lion.
http://support.apple.com/kb/DL1482
Click to open the dmg but do not attempt to install the pkg.. it won't work anyway.
2. Download and install unpkg.
http://www.timdoug.com/unpkg/
Run unpkg on the desktop.. it is very simple.. drag the AirPortUtility56.pkg file over to unpkg.. and it will create a new directory of the same name on the desktop.. drill down.. applications utilities .. there lo and behold is Airport utility 5.6 .. drag it to your main utilities directory or just run it from current location.
You cannot uninstall version 6 (now 6.3 if you updated) so don't try.. and you cannot or should not run them both at the same time.. so just ignore the toyland version.. the plastic hammer.. and start using 5.6.. a real tool.
For screen shots see this post.
https://discussions.apple.com/thread/4668746?tstart=0