Oh, I know this one!
See, you have to either have something called a Moo Flute, or a Moo Radio that plays the Moo Flute theme. After playing it, the Moo will wake up, and attack you in a tired and depressed rage, so then you have to cheer the Moo up. After a while of this, it will have been cheered up, and politely leave.
...Was that right? It felt right, right?