Unfortunately, AirPort Utility.....the application that you need on your Mac to set up and configure the AirPort.....does not support the very old version of the AirPort Express that you have. Specifically, Apple dropped support of the older AirPort models about 3 years ago.
The good news might be that if you have.....or, could borrow for 10 minutes..... an older Mac that is running an older operating system like Leopard or Snow Leopard, or a PC with AirPort Utility for Windows installed on the device, you can use that computer to set up and configure your old A1084 version of the Express.