No account. No key. Nothing leaves your Mac. Quit Ableton Live before you start, and you'll be reading a mix in ten minutes.
Live Suite, or Standard with Max for Live. Live 12.
Apple Silicon. Apple menu → About This Mac says M1, M2, M3 or M4.
The zip from your download link, and Terminal, which is already on your Mac.
Recorded on a clean Mac, from the lock form to the Box on the Master. About six minutes.
Quit Ableton Live. Unzip the download. Open Terminal (Spotlight → type Terminal). Type bash and a space, then drag install-mixstein-engine.sh from the unzipped folder into the Terminal window, and press Return.
It installs the engine and drops the small AbletonMCP script into your Ableton User Library. That script is how the engine talks to Live.
Open Ableton Live → Settings (Preferences) → Link, Tempo & MIDI → under Control Surface choose AbletonMCP. Input: None. Output: None. It stays selected from now on.
Drag MIXSTEIN-box.amxd from the unzipped folder onto your master / mix bus track. It finds the engine on its own, and the engine talks to Live through the script you picked in step 2.
Hit Read the mix bus. Then Find what's fighting. Read what it heard.
Your User Library lives somewhere other than ~/Music/Ableton/User Library. Find the real path in Live: Settings → Library → "Location of User Library". Then re-run step 1 with that path in front:
ABLETON_USER_LIBRARY="/Volumes/Audio/Ableton/User Library" bash install-mixstein-engine.sh
A folder you already had there is kept as a copy under ~/.mixstein/backup/ and put back if you uninstall.
bash install-mixstein-engine.sh --uninstall with the same ABLETON_USER_LIBRARY=… in front if you used one to install. It leaves your Mac the way it found it.
Ask in the pinned install thread in The Board Room. Every answer is public on purpose so the next person doesn't have to ask. Paste what Terminal said, and what About This Mac says.