Are you plugged into the TC though or something else?
Sometimes you just need to start over.. reset the TC and do it again..
I think you will do better loading the older 5.6 version utility.
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 6.1 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
Then you can follow exactly the apple setup documents.
These have not been updated for the latest version of airport utility.
http://support.apple.com/manuals/%23airport
Or ring apple support and get them to talk you through it.