Have you tried putting an actual CF card in the reader ? And have you pushed the More Info button on the About this Mac window, which brings up system hardware info, and lists all the hardware specs and devices connected to said G5? Do you see Sandisk Imagemate under one of the USB port brackets?
As any Mac user would know the drive doesn't show up on the desktop unless you put something into the drive. This goes for CD/DVD-ROM, Floppy drive, Zip drive, and of course, flash card readers.
And no, you don't need to install any drivers. I use the same card reader on my Mac dual G4 MDD running Jaguar.