I may be wrong, but it seem like your Mac should "seeing" the printer with or without the drivers installed. The PPD file you downloaded is needed for the computer/printer to talk effectively, but not to "detect" the printer.
I would likely blame the printer.. Sometimes those machines don't actually support USB printing, whether or not it has a port. Or maybe USB printing could be disabled in the Admin settings of the machine??
Is there a reason your not trying to print over the network wired or wireless? That is your best bet.. If you still can't automatically detect it on the network, but you are sure the machine is connected to the network, you could try to add the printer manually with the IP address of the machine.
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If you are really interested in figuring this thing out.. here is the
manual.