Using Old Airport Utility 5.6.1 with New MacOS (mavericks & newer)
This has probably been answered elsewhere but I couldn't find it by searching. This isn't a question, it's an answer. If you still have very old hardware that is not supported by the newer versions of Airport Utility, then you're unable to configure or manage those devices, even though the devices may still work just fine. For example, I have a very old Airport Express that I use to stream music to my stereo receiver. It ***** that Apple would do that, especially since the fix is so easy. To manage these old devices, you need to use an old version of Airport Utility, version 5.6.1, which I'm sure got deleted when you installed Mavericks, Yosemite, or El Capitan. So first, you need to find a copy of AU 5.6.1 and put it on your computer. Hopefully you can find a site that has a copy for download. Or perhaps you have an old disc or backup somewhere.
But then you won't be able to open that utility because it's incompatible with the newer OSs. However, a fine gentleman has created a launcher that will run a small script that opens AU 5.6.1. Search for Airport Utility 5.6.1 Launcher and Mr. Corey J. Mahler explains it all there. (I can't link to his site or this post will get deleted.)
<rant>I've stuck with Apple products for a quarter of a century, but I really hate it when they force up to buy an expensive upgrade when there's no need, or they drop support or features for hardware/software that still works perfectly fine. I mean, I can understand that you can't make a new, modern OS run on a very old computer, but some of the forced upgrades just don't make sense. Why can't the new Airport Utility just support the old Airport Express? Or if an amateur programmer can so easily make the old utility work, couldn't Apple do the same?</rant>
MacBook Pro with Retina display, OS X El Capitan (10.11), null