Thanks for the clarfication.
AirPort Utility 5.6 installed on Mountain Lion will certainly work with an A1084 AirPort Express, as I've just tested this out again here with an older A1084 that I had on hand.
If you have not already done so, you might want to try a Factory Default Reset on the AirPort Express and try again.
Power off the AirPort Express
Wait a few minutes
Hold in the reset button first, and keep holding it for an additional 9-10 seconds while you simultaneously plug the AirPort Express back in to power
Release the reset button after the hold period and allow a full minute for the Express to restart
Click the fan shaped AirPort icon at the top of the Mac's screen and look for a wireless network with a name of Apple Network xxxxxx
Log on to that network....no password is required....then open AirPort Utility 5.6 to see if it will recognize the Express
You can also temporarily connect an Ethernet cable from your Mac to the Express. Once you have the Express configured, disconnect the Ethernet cable.