So forget that previous post of mine...
Have you got In1 and In2 set high and low via pins 8 and 9 to give it direction?
Edit... and this is me never having used the MM as a shield, always loose... don't you need to give the MM 5V and ground at the screw terminals to drive the board?- or does it take power from the Arduino underneath? (The Vin is the motor power and not to drive the MM)