Well, I thought that normally when you're using a third-party USB-MIDI interface, that if you have any trouble with the device not being seen correctly then you need to install the driver that goes with the interface, not the driver for the device. For instance, if you were using an M-Audio USB-MIDI interface then you would want to use the driver that goes with the M-Audio interface, not the driver that goes with the device (in this case, the Yamaha Clavinova).
Note that many USB-MIDI interfaces are supposed to be "plug-and-play" and "class-compliant," so they might not even have a driver available, or you might need to search carefully on the manufacturer's web site for the driver you need.
However, recently there was a post (either here or in another forum) from someone who was having trouble using a Yamaha keyboard with a third-party USB-MIDI interface, and in he said that he finally got it working after installing Yamaha's USB-MIDI driver, which was why I suggested that you might try that-- especially since you had said that you couldn't find a driver for the interface.
You didn't mention what brand of USB-MIDI interface you bought, but I should warn you that some of the inexpensive ones from companies that seem to have no web site turn out to be unreliable. It's almost always better to spend a little more money on a name-brand USB-MIDI interface from a well-known company that has an established web site and readily-available USB-MIDI drivers in case they're needed. So if all else fails to help, I would suggest getting a different USB-MIDI interface to try.
Another thing you might try is see if you can get the existing USB-MIDI interface to work with a different USB port on the computer. These days a lot of computers are equipped with USB 3 ports, but some equipment is not able to connect successfully to a USB 3 port. If you're currently trying to connect to a USB 3 port, see if moving the USB-MIDI interface to a USB 2 port makes a difference.
And if you're already connecting to a USB 2 port, try moving it to a different USB 2 port. I've run into situations where some device (such as a computer keyboard or a USB flash drive) wouldn't work correctly when plugged into a particular USB port on my computer, and moving it to a different USB port resolved the issue. You might think that one port's the same as any other, but each has its own ID within the computer, and sometimes it can make a difference which one you plug a device into, such as if any driver information associated with a specific port got mucked up somehow.