I verified that DIGITAL KEYBOARD was showing up as an audio device as well. (Interestingly enough, it was listed as just DIGITAL KEYBOARD, without the '-1' on the end, when I was using it as a class-compliant USB-MIDI device before installing the driver.) I started my DAW again and verified that DIGITAL KEYBOARD-1 showed up as a MIDI device. After the installation was over, I reconnected the USB cable to the computer.

At first the installation acted like it wasn't doing anything- the progress indicator didn't even start moving for a while- but once it finally got going it installed quickly. (I prefer to leave the USB cable plugged into the back of my PSR-EW400 to minimize the number of times I plug in and unplug the cable, to avoid any unnecessary 'wear and tear' on the port.) I unplugged the USB cable from my computer. I installed the latest version of the driver- but I didn't follow the installation instructions exactly in particular, I did not turn off my computer first as requested in the instructions. Okay, the computer wouldn't see the PSR-EW400 as an audio device without the driver installed. Likewise, if the situation were reversed and you were trying to install the Steinberg driver with a Yamaha keyboard that has USB-MIDI but not USB-audio, I think you would also get an error, because the Steinberg driver is programmed to expect both types of data in the USB signal and would not be able to handle the lack of audio data in the USB signal. So you need the Steinberg driver, which is programmed to handle both types of data. I don't think the Yamaha driver can understand the combined audio and MIDI data- it isn't programmed to pick out the MIDI data and ignore the audio data- which is why it can't detect the keyboard as you're expecting. The PSR-E363's USB connection carries audio and MIDI data simultaneously, and the driver needs to be able to understand the data coming through the USB connection. I think the Steinberg driver is for both USB-audio and USB-MIDI, while the Yamaha driver is for USB-MIDI only. I think that with the PSR-E453/PSR-EW400 as well as the PSR-E363/PSR-EW300/YPT-360 you're supposed to install the Steinberg USB driver, and only that driver, not the Yamaha USB driver.

Yamaha usb midi driver mac