Sorry… I looked inside and I see what you mean.
This is a standard case of the younger employees at Windows-centric Roland knowing little or nothing about "vintage" Macs.
Here's the details:
* The driver Roland provides alone should make it easily seen by OMS on a serial machine.
* On a USB-only machine, you also need the USB OMSMIDIDriver extension .
* If the USB OMSMIDIDriver extension is present in System/Extensions The Edirol should be "seen" and should work.
* If you installed OMS originally on a USB-equipped machine using the OMS installer, it should have detected that and installed the USB OMSMIDIDriver extension.
If you don't have the USB OMSMIDIDriver extension for some reason, I've attached a copy. You can simply drag it into the System/Extensions folder and reboot. That should get it going.