Applies to Launchkey MK3
Go to the Preferences (Options > Preferences on Windows, Ableton Live > Preferences on macOS) and go to the Link/Tempo/MIDI tab:
Ensure to select "Launchkey MK3" for the Control Surface:
and for the Input & Output select the second port listed:
Note that on Windows systems the DAW port will appear as "LKMK3 MIDI (Port 2)".
Your settings should match those shown here:
You'll know that the Launchkey is working with Live correctly when you see Session + Device/Volume/Pan/Sends pads light up when you hold down 'Shift'.
Incorrect settings may cause issues like certain notes not sending MIDI.
Note that enabling MPE for the input port means that all MIDI channels are in use, so it's not possible to set a track to discretely respond to one MIDI channel. You may wish to disable it if not using Polyphonic Aftertouch data from the pads.