In order to use your keyboard's onboard sounds and MainStage, do one of these:
- Use the hardware's built-in output jacks for the onboard sounds and your Mac's interface/headphone jack for the software sounds. You would have potentially 4 channels if running both in stereo, 2 for the hardware keyboard and 2 for the MainStage sounds.
- Run the hardware's built-in output jacks into your audio interface’s inputs, and then blend the two (hardware sounds and software sounds) using the mixer knob on your interface.