Looking at the Owner's Manual, it appears that the P-85 has MIDI IN/OUT ports, not a USB-MIDI port.
That means you most likely don't even need to use a USB-MIDI driver.
If you're using either a MIDI-to-USB cable or USB audio interface that has MIDI ports to connect your P-85 to your computer but the software doesn't recognize that a MIDI device is connected-- and note that it probably would not identify the device as a Yamaha P-85-- then you might indeed need to install a USB-MIDI driver.
However, in that case it would need to be the USB-MIDI driver that was written for the MIDI-to-USB cable or USB audio interface, so you wouldn't be able to install Yamaha's USB-MIDI driver unless you were using Yamaha's UX16 MIDI-to-USB cable.
Most MIDI-to-USB cables and USB audio interfaces are designed to be MIDI class compliant, so they normally work without needing a USB-MIDI driver.
What are you using to connect your P-85 to your computer?