kap

Thanks, Brad! (so it is unlike my old numacompact, okay) In that case, there are some options. Should I take the output from the headphone jack and run it into the PX-560 Audio-In Jack? Or should I connect a USB-to-audio cable to the Audio-In Jack (and tell the mac somehow to send sound out that way)? Or do I need to buy an audio interface?

This is partly working for me - what is not working is to get the mac to route the sound back through the USB cable to the keyboard so that Mainstage sounds can come from the same output (and not just from the Mac headphone jack). On other keyboards, there is an output choice automatically listed on Mainstage, but I do see the PX-560 there. Anyone know a way to make Mainstage send the audio output back via the USB cable to the PX-560?