OS X 10.8.2 no longer supports the older versions of the AirPort Express.
Please take a look to locate the model number of your AirPort Express. It is on the side of the device.
If you see A1084 or A1088, you have an older version of the AirPort Express that OS X will not support. In that case you may be able to install more software on your Mac to allow it to work with the older AirPort Express.
See this article for details: