So: download this file and place it on an audio track. In Live’s preferences, select the Launchpad as a control surface. On the effect, select the Launchpad from the upper menu, then Button_Matrix from the lower one, and then hit the button.
Then it should work when you flip to "user 2" mode for the launchpad. Session and Mixer modes work normally.
There is a comment in the file (if you open it for editing) that I should have taken out about "change this number is you want to use user 1 mode instead". That actually doesn’t work because User 1 mode doesn’t transmit the necessary messages; leave it as user 2.