If the drive is not being recognized by the AirPort Utility, it can't be shared out and why you are not seeing it in the Finder. The USB port on the AirPort can support USB bus-driven drives, but may require a powered USB hub be used if the drive draws more power than the port can supply.
Okay good information. I'll try a powered HD or connect the one I have to a powered hub and see if that works. Thanks for your help.