I have an Airport Express 1088 (about 7 years old). My Macbook with Mountain Lion 10.8.4 doesn't sees it, nor does my wife's Macbook. Strangely, my iPad and iPhone can detect it and the network traffic goes through it, and everything works smoothly. However, the Airport Utility (version 5.6) on my Macbook does not see it.
Additionally, I can play music from my iPad or send the YouTube voice channel to the Airport Express, so the device works, but my Macbook still doesn't see it.
I have a MacBook (13-inch Aluminum, Late 2008) but I don't think that matters. I think that the OS version will matter here, because two weeks before it worked well.