Airport Express A1392 - trouble with set up

it wasn't discoverable trough my computer

What actions did you take to determine that it wasn't discoverable?

You say you acquired the AEX. Was it second hand? How were you expecting to identify it? Was it already reset to factory settings or did it have an established name and set of credentials?

As you haven't found it reset it to factory settings by holding in the reset button with a pin or a paper clip for ten seconds and follow the instructions on page 19 of Apple's manual. They also provide detailed instructions in the support page called How to reset your AirPort base station and you will be following the instructions for a Hard Reset.

After the reset it should appear in the list of networks, or in Airport Utility via the 'Other Wi-Fi Devices' button on the left. The linked page says "The base station network name is set to "Apple Network", followed by the last six digits of the base station's MAC address."

Then follow the prompts to set it up for music streaming.

We will assume that the AirPort Express is not defective and that you are using a WiFi router to provide your WiFi network

Power up the AirPort Express for a minute or two

Hold in the reset button on the back of the Express for 7-8 seconds and release

Allow a full minute for the Express to restart to a slow blinking amber light status

Click on the WiFi menu at the top your Mac's screen

Look for a listing of New AirPort Base Station

Just below that listing, click directly on AirPort Express

When you click on Airport Express, that will start up the AirPort Utility setup "wizard". The wizard will take a few seconds to analyze the network signal and then suggest what it thinks will be the correct setup for the Express based on what it "sees". It is likely that the Express will be set up to "Join a wireless network"

If your "main" router is also another Apple AirPort router, the Express will be set up to "Extend a wireless network"

Follow the prompts