Try this:
go to:
http://www.apple.com/support/downloads/airportbasestationupdate2007001formac.html
Mount the dmg.
Open the mounted drive in Finder.
Right click on AirPortBaseStationUpdate2007001.mpkg and select Show Package Contents
Open the "Contents" folder
Open the "Packages" folder
Right click on AirPortUtility.pkg and select Show Package Contents
Open the "Contents" folder
Double Click on Archive.pax.gz - this will extract the files from this archive, and on your desktop you should have a folder called "Applications". Inside this folder will be the "Airport Utility".
Copy it over to your Utilities folder inside of Applications and you'll be all set.
After you've done this you'll want to run Software Update, which will grab the latest version, since you just extracted the "base" version.
Let me know if this works for you, it's worked for me on several computers since I lost my disk.
