This version of AirPort Utility doesn’t support this base station

Check the model number of the AirPort Express on the side of the device.

If you see A1084 or A1088, you have an older version of the Express that is no longer officially supported by Apple's newer operating systems in Lion and Mountain Lion.

See this thread for some ideas and a possible workarounds:

Re: Airport time capsule and Airport Express model A1088

what if you have an A1034 and Mac os x mavericks how is one supposed to get it to work? or an A1034 and windows 7?

The A1034 can be configured by a Mac running Leopard (10.5.x) or Snow Leopard (10.6.x). A PC running Windows 7 could set up the A1034 as well.....as long as AirPort Utility 5.6.1 was installed on the PC.

http://support.apple.com/kb/dl1547

I have Airport Express 1088 plus an Airport Extreme. Extreme is in main area and wired with Ethernet to Express. True Airport Utility doesn't support the older Express but they work together just fine and extend WI-FI to another area.

We did not say that things would not work with an older Express. We said that the newer version of AirPort Utility cannot "see" the older Express in the event that any changes need to be made to the Express.

Things will be OK as long as you do not need to make any changes to the AirPort Express. But if you do, then your current version of AirPort Utility will not be able to "see" the AirPort Express, and as a consequence you will not be able to configure the Express with any new or different settings.....unless....you have an older Mac or a PC running an older version of AirPort Utility.

An A1084 version of the Expess is at least 8+ years old, and as much as 10+. The average useful life of this model was about 3-4 years, so it is remarkable that the Express is still functioning. Congrats on your good luck.