Download the software from the Apple website instead of using the CD. http://support.apple.com/kb/DL1664
Although, as mende1 mentions below, you shouldn't have to install anything, it should set itself up automatically.
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You don't have to use your AirPort DVD, because your Mac already has got the apps that you need to set up your AirPort Express.
Just plug your AirPort Express and it will turn on. Then, open AirPort Utility (in /Applications/Utilities) and follow the steps. It will be detected by AirPort Utility and your computer will connect automatically to it. You don't need to download anything unless you haven't updated AirPort Utility, but as you are using OS X 10.8.4, you should have to latest AirPort Utility version