It shouldn't need a driver. The U-20 is purely a MIDI equipped ROMupler
(sample playback) keyboard.
It's no 'sound canvas' so trying to use such an editor will not get you
far. Don't even think the U-20 had a GM mode.
If you are using Cakewalk, it sounds to me if you are referring to a
definition file. This lists the presets, making patch selection from
Cakewalk easier as what is on the screen matches the LCD of the U-20.
I'm unable to access Cakewalk at the moment, so I'm not sure if there's
a def file in there or not.
If there is, I'm sure it will be in the Roland folder. I also believe
the U-220 is the same beast minus the actual keyboard.