Here are a few things to check before you begin performing:
Make sure your MIDI controllers, instruments, microphones, and other music equipment is connected to your computer and is working.
Test the audio output from MainStage using the audio interface and speakers or monitors you plan to use in performance.
Select a patch with a software instrument channel strip and play your keyboard controller. Watch the Activity Monitor to make sure MainStage is receiving MIDI input from the controller, and make sure you can hear the audio output.
Make sure any instruments or microphones you plan to play through audio channel strips are connected to the correct audio inputs on your audio interface. Select a patch with an audio channel strip and play or sing to make sure you can hear audio output.
For the best results, close any applications that you do not need while performing, particularly applications with high processor or RAM requirements.
Disconnect the computer running MainStage from any network connections.