You can setup a livestream with Zoom using Live Mixer for Sunday Keys, and there are a few ways to do it.
The easiest way to route audio from MainStage to Zoom is going to be via Zoom's ability to share your computer's audio directly to Zoom. You will just need to make sure that the Audio Output device selected in MainStage is set to "Built In" Output. You can then use your headphone output jack to monitor audio.
Another way to accomplish this (if you want to monitor audio via an audio interface) is to use a "virtual audio cable" type of software such as Blackhole (a virtual audio cable). Blackhole is free and after installing it on your Mac, you will want to select "Blackhole" as the Audio Output device in MainStage. Then you will select "Blackhole" as the Audio Input device in Zoom. Check out this article for more.
The only downside with this simple routing setup is that you will not be able to monitor your MainStage audio (besides any monitoring solutions in Zoom). So, if you want to monitor with an audio interface, you will need to check out this video on setting up an aggregate audio device. Your aggregate audio device would be your audio interface and Blackhole. Click here to learn more.
Then you will send all of your audio to the Blackhole outputs (to get the audio into Zoom). Next you will setup bus sends on all of the channel strips that are sending audio and route them 100% to an empty bus (like bus 15). Then on that auxiliary bus channel strip, you will choose the output to the audio interface outputs. This will be your monitoring audio feed.
Here are some additional steps to follow inside Zoom in particular to check settings:
1. Start a Meeting
2. Screenshare your desktop once (make sure you choose "optimize screen share for video clip". You should find this option when choosing screens) - This will allow Zoom audio driver to install "zoomaudiodevice"
3. Close meeting
4. Go to MainStage -> Preferences -> Choose output as zoomaudiodevice
you can now test the setup by opening another meeting with the screenshare steps 1 and 2.
5. During the live meeting, you can choose another screen as long as the optimize option is active.
6. If you need to route mic, add a audio channel on MS and choose your input. (if you're getting feedback, make sure you mute the built-in output via Audio Midi Setup on mac or use a headphone)
For all the installation and tutorial videos for Live Mixer go here.