The audio and MIDI devices to be used by MultitrackStudio can selected in the Studio menu's Devices window.
MultitrackStudio Pro supports up to 140 audio channels, up to four MIDI In devices and up to four MIDI Out devices. Lower editions support up to two audio channels, one MIDI In device and up to two MIDI Out devices.
On Mac there's just one audio driver type: Core Audio.
The Audio Output Control (available from the Studio menu) can be used to set the properties (channel selection, levels etc.) of the audio output device. The controls depend on the audio driver type and devices used.
If an MPE keyboard sends RPN messages to change the MPE configuration, the MPE check box will be updated accordingly. MultitrackStudio may not even be running when your keyboard sends these messages, so you may have to switch it on yourself.
Note: do not use the MPE option if you don't have an MPE keyboard. There's no gain but many potential problems.
In most cases the incoming MPE data will go to MIDI channel 1. If an MPE keyboard sends a different configuration it could go to MIDI channel 16 or both 1 and 16 (two MPE zones).
The Options button in the MIDI Out Devices section pops up the MIDI Out Device Options window. It contains settings regarding sync code and recording connections.