Computers know a lot of things, but require help sometimes 'seeing' a device. This is true with many musical pieces of gear. You need to install what is called a "driver". If you tell us what type of computer we'll tell which driver you need to get the job done.

You can go to the official Yamaha download site and get the appropriate MIDI driver for the reface

Http://download.yamaha.com

Select your country
Type in the MODEL NAME
when the SEARCH RESULTS appear confirm your model by clicking on the Name

A list of downloads for the reface will appear. Find the USB-MIDI Driver for your computer type and operating system.

The download will include the instructions.