Let's make sure that you really do have a 1st Gen AirPort Express.
Locate the model number on the side of the Express. It is hard to see with gray lettering on a white background, so you may need a magnifying glass and good lighting.
The model number of the 1st Gen is A 1264. If you see A 1084 or A1088, these are much older versions of the product that were offered long before Apple began to use the "Generation" naming convention in 2008.
If you happen to have A 1084 or A1088......you won't be able to administer the Express with a Mac running Yosemite. You will need to have a Mac running the Leopard or Snow Leopard operating system.....or a PC with AirPort Utility installed on the computer to be able to administer the older versions of the AirPort Express.