If you read on Apple's discussions forums you'll see a lot about this. Try this first. Mount the disk on your mac, then eject it. Without turning the drive's power off, plug it into the AEBS. If it is formated FAT32 or HFS, you should be able to see it in the airport utility and it should mount on your mac.
Do you have a password set for drive access in the airport utility? Do you have the AirPort Disk Utility installed locally on your mac? Is it set to automatically discover AirPort disks?
AEBS have trouble with some USB hard drives, some need to be directly plugged in, some need to be mounted on a mac first, some just work. I have a Lacie 500GB Porsche drive that I have to have powered on first, then plug into the AEBS and then power on the base station for it to work. Otherwise I have to mount it on my mac, unmount and then plug it in.
Apple's implementation is not great, but with some fiddling, most can get it to work. A few drives, mainly USB/Firewire type drives, don't work at all. YMMV