Is there an Arduino motor shield library?
From Chrisl's comment: https://github.com/gallingern/arduino-motor-shield-r3

You don't really need a library though as the code is simple enough:

void setup() {

  //Setup Channel A
  pinMode(12, OUTPUT); //Initiates Motor Channel A pin
  pinMode(9, OUTPUT); //Initiates Brake Channel A pin

}

void loop(){

  //forward @ full speed
  digitalWrite(12, HIGH); //Establishes forward direction of Channel A
  digitalWrite(9, LOW);   //Disengage the Brake for Channel A
  analogWrite(3, 255);   //Spins the motor on Channel A at full speed

  delay(3000);

  digitalWrite(9, HIGH); //Engage the Brake for Channel A
}


From here.