Let's make sure that you have a 1st Gen AirPort Express......
Please locate the model number on the side of the Express. It starts with an "A" followed by four numbers.
A 1st Gen AirPort Express is model number A 1264. Is this the model that you have?
Next, we need to know what version of AirPort Utility that you are using to try to configure the Express.
Unfortunately, AirPort Utility 6.3.1 does not support the older A1084 version of the AirPort Express that you have.
If you have another Mac that is using an AirPort Utility 5.x version, or a PC, then you could use that computer to administer the older AirPort Express that you have.
The 1st Gen AirPort Express....model A1264....and current 2nd Gen AirPort Express....model A1392.....would both work with AirPort Utility 6.3.1.