The USB MIDI driver relates to connecting the KP3 by its USB connection. You don't need drivers to connect by MIDI.
I don't have a KP3 but it sounds like the editor requires the KP3 to be connected by USB, not MIDI (and there is no point in doing both because they provide more or less the same functionality otherwise). Someone else may be able to confirm if that is correct?
When you install the driver and plug in USB, you also have to run the utility it puts in:
Start > Programs > KORG > USB-MIDI Driver > Install KORG USB MIDI Device
that utility assigns the driver to the device, otherwise windows continues using its own incorrect driver.
Current Gear: Kronos 61, RADIAS-R, Volca Bass, ESX-1, microKorg, MS2000B, R3, Kaossilator Pro +, MiniKP, AX3000B, nanoKontrol, nanoPad MK II,
Other Mfgrs: Moog Sub37, Roland Boutique JX03, Novation MiniNova, Akai APC40, MOTU MIDI TimePiece 2, ART Pro VLA, Focusrite Saffire Pro 40.
Past Gear: Korg Karma, TR61, Poly800, EA-1, ER-1, ES-1, Kawai K1, Novation ReMote37SL, Boss GT-6B
Software: NI Komplete 10 Ultimate, Arturia V Collection, Ableton Live 9. Apple OSX El Capitan on 15" MacBook Pro
you can do both usb and midi, you can change in the settings on the software. the message you got is a warning that the driver is not installed properal. i found i had to installed the driver twice to make it work. plus if you dont use the same port ever time youl have to re-install the driver ever time you use a new usb port on your computer.