Thanks to everybody for posting on this thread. After a torturous couple of days with a deadline looming, I was trying to get my HP DesignJet 500PS to plot from Panther and VWKS 10.5, I scanned this thread and went to the Gimp Package site, 500 driver update and USB updates, and they work....no speedy, but they work.
Install the driver first, then the USB updater to that it can 'discover' the driver when setting up the printer.