Are you sure the MM gets its power from the 20V?.... I'm pretty sure that 20V is just fed to the 298's pin that drives the motors, and that it needs a different supply of 5V to drive the logic side of the 298 chip itself. The manual lists Vlogic and Vsupply as separate items....
Certainly, when I ran mine (again, loose not as a shield) I took 5V and ground from the Arduino to the 5V and ground screw terminals on the MM board.