I downloaded the USB-MIDI driver for the YPG-235 from Yamaha and installed it.

The driver is listed under Hardware and Sound in the Control Panel and the keyboard itself is listed as Yamaha Portable G under devices. If your USB MIDI device is not being recognized by your computer, consider the following: Windows. USB-MIDI Driver V1.3.1 for Mac OS X 10.4.11. This guide covers basic troubleshooting steps to determine whether your USB MIDI device is recognized, and working properly on a Mac computer. MIDI devices should appear in the Sound, video and game controllers category. The following version: 1.2 is the most frequently downloaded one by the program users. However when I do this exact thing on my old macbook this box does not appear. If your USB MIDI Controller is not working on a Mac please check the following:-Make sure the Controller is connected directly to the USB port on the Mac-Open the Apple Menu > About this Mac > More Info > System Report >USB and check that the Controller appears here in the USB Device Tree. The MIDI Setup in particular can come in handy if you begin to experience limited or no response from your MIDI device or if the device is not recognized at all.