do you have a link to it? if its the motor shield i'm thinking of: http://webzone.k3.mah.se/k3dacu/arduino/shields/motor_control/
then for each motor it has a DIR and a PWM input. setting the DIR pin high makes it go one way, and low makes it go the other way. With the PWM input, you do an analogWrite() to that pin to set the speed of the motor. It looks like its also setup to do encoder inputs, but I dont know how that works, so I cant help you out much. i dont know what pin numbers it plugs into, but it should be fairly easy to figure out.
I've been looking at this board as a reference design, as I plan on making some motor controller boards based on it. They aren't going to be shields, but they will have the same interface. I'll post more on here when I have more info on that.