Here's a version for more Mac/Mainstage-clueless people such as myself... this is against 3.5.3.
1.
Open the "Audio MIDI Setup" app (outside of MainStage) and create an Aggregate Device with all of the audio outputs and inputs you will need (e.g. where you will listen and where your pad(s) will be plugged in.) Rename the device to something non-generic to avoid automatic renaming if you need to add another aggregate device later.
2.
Go to "MainStage > Preferences > Audio..." and set
first "Audio Output" and
then "Audio Input" to your aggregate device's name.
3.
Add an 'Audio Channel Strip' - note that these seem to remember the devices that were available at the time of creation, so if you need to add additional inputs to your aggregate device later, you may need to delete and re-create this channel.
4.
Set its input to the input you plugged your pad into. If you have a pad with stereo output such as a 2-piezo mesh pad, the stereo input will look like "Input 1-2" or "Input 3-4". Most built in line ins are stereo. Super cheap USB adapters are usually mono. This is the cheapest likely stereo line in device I could find: heaven2017 USB 2.0 6 Channel 5.1 Optical Audio External Sound Card for Notebook Laptop PC (Blue)
https://a.co/d/gLrHDcX (or anything that looks roughly like it, if you like name brands there is a StarTech device for around $30 I can confirm works.)
5. In the channel you added, click the blank box in the
"Audio FX" row and select "Audio Units > Audiofront > DSP TRIGGER > (Mono or Mono->Stereo)" - I think you want "Stereo" as that is what "Drum Kit Designer" has available.
6.
Add an 'Instrument Channel Strip'
7.
Set its input to 'Drum Kit Designer' - I think you want the "Stereo" option.
8.
Click on the large drum pad in DSP TRIGGER to confirm that the MIDI is being sent to Drum Kit Designer.