I have an original AirPort Express 802.11g that I still use in the house, but for whatever reason, Apple has updated AirPort Utility so that it no longer works with that older unit. Pretty frustrating, because it otherwise works fine.
It used to be that you could just download AirPort Utility 5.6.1 straight from Apple, but that only worked up until Snow Leopard, officially (and up to Mountain Lion, unofficially). However, Mavericks has now broken it entirely.
Thankfully, it seems Corey J. Mahler has figured out that it's a new system framework that has broken the old app, and has created a launcher that will inject the old framework as the old app launches, allowing it to launch and manage the old AirPort base station as before.
I admit, I'm not the most experienced developer, but it looks like he injects it with an AppleScript (copied below) that launches a shell script that does the injecting. Seems fairly harmless, and should only affect the 5.6.1 app. (I welcome corrections, if that's not the case.)
tell application "Finder"
set currentDir to POSIX path of ((container of (path to me)) as text)
end tell
set currentDir to quoted form of currentDir
set lib80211 to quoted form of "AirPort Utility 5.6.1 Launcher.app/Contents/Resources/Apple80211.framework/Versions/A/Apple80211"
do shell script "
export DYLD_INSERT_LIBRARIES=" & currentDir & lib80211 & "
cd " & currentDir & "AirPort\\ Utility\\ 5.6.1.app/Contents/MacOS
./AirPort\\ Utility"
Anyway, check it out for yourself here.