:EDIT: I have just tried this again and although I can "join a wireless network" with the Airport EXTREME, I cannot get it into client mode which would give out internet or LAN to the ports on the back. bummer. So I'm thinking this tool is only really good for the old Airport Expresses that no longer work with the new Airport Utility 6:
With just a bit of help from a script that opens the old Airport Utility 5.6 on your Mac using OS Mavericks or above.
Download from here:
First rename your current Airport Utility to something like Airport Utility 6
Then (in this order) drag the utility launcher to your Utilities folder.. authenticate
Then Drag Airport Utility 5.6 to your Utilities folder .. authenticate
Then double click on the launcher (the one you previously dragged to your Utilities folder)
It will open Airport Utility. In the wireless section hold down the "option" key and "join a network" will be there.
How the Launcher Does What It Does
With the release of OS X 10.9 “Mavericks” Apple updated file Apple80211 (contained in /System/Library/PrivateFrameworks/Apple80211.framework/Versions/A). Whatever the other effects of this update, it broke backwards compatibility with older versions of AirPort Utility. Thankfully, the fix is simple: Inject an older version of the file when launching older versions of AirPort Utility. This is precisely what the Launcher does.
When you run the Launcher, it simply injects the OS X 10.8 “Mountain Lion” version of file Apple80211 (which the Launcher contains) and then runs AirPort Utility 5.6.1 (which is included in the download of the Launcher). That’s it. One file is the difference between AirPort Utility 5.6.1 running and throwing an error under more recent versions of OS X.