Q: Installing a first gen airport express with a mac using latest version of airport utility?

David,

Check your AirPort Express and look on the side of the device for the Model No. 

A 1st Generation Express will be Model No A 1264.....and it can be configured with virtually any version of AirPort Utility offered in the last 3-4 years.

What Model No do you see there?

What operating system are you using on your Mac or PC?

This is an older version of the Express that was sold long before Apple started naming generations.

Download and install AirPort Utility 5.6 for Mac OS X Lion

You can keep both AirPort Utility 5.6 and 6.1 on your Mac and use the one that you need.  AirPort Utility 5.6 will also work with your newer AirPort devices as well.

What happens if you're running Mountain Lion? I can't install Airport Utility v5.6 while running Mountain Lion and I also have an older Airport Express that Airport Utility v6.1 doesn't see.

What happens if you're running Mountain Lion?

Apple stupidly will not allow you to install AirPort Utility 5.6 on Mountain Lion in the normal way. It's more or less a way of telling you that they will not support your older AirPorts any longer.

Check this article out for a workaround:

http://www.macworld.com/article/1167965/mountain_lion_and_the_ancient_airport_ba se_station.html

This is not supported by Apple, so up to you to decide if you want to do this.