To unlock each ending, you must finish the game a certain way. For ending M, you'll need to:
Optional: You can start at Civitavecchia (level 2 below).
Level 1 - Keep left.
Intermission 1 - Leave Olga (say "I am alone").
Level 2 - Keep right.
Intermission 2 - Choose "Shut up, lets take a look around Gorilla".
Level 3 - Lose the race.
Finale - Nothing.
Video courtesy of Dernop