It sounds like you want to setup your Airport Express to Airplay music from iTunes. I found an article with steps to help you get this setup:
Connecting to an existing Wi-Fi network, exclusively for iTunes music
AirPort Express used exclusively for AirPlay by setting it up in Client Mode.
Choose this configuration if the computer containing your iTunes library may be out of direct range of the AirPort Express, but is within range of the Wi-Fi network.
How to Set It Up
- Connect your AirPort Express to your home stereo or powered speakers.
- From your computer, join the wireless network. Use the AirPort status menu in the menu bar on a Mac. On a Windows-based computer, hold the pointer over the wireless connection icon until you see your AirPort network name (SSID), and choose it from the list if there are multiple networks available. Enter the network password, if necessary.
- Open iTunes on your computer and choose the AirPort Express(es) you wish to stream music through from the speakers pop-up menu in the lower-right corner of the iTunes window.
I also recommend reviewing the sections titled "Some things you should know about AirPort Express with AirPlay" and "Things to consider before you begin" in the full article:
Thank you for posting in the Apple Support Communities.
there is a discussion on here https://discussions.apple.com/thread/4172563?start=0&tstart=0
OR if you don't want to use terminal there is how to do it here http://frank.is/mountain-lion-and-the-old-airport-utility/
not sure i can link "non apple" pages here but google "how to install airport utility 5.6.1 on mountain lion" will be the 3rd option
of course once you install airport utility it will say there is an uptade you have to say NO