Arduino stepper

Question

I am building a system with the Arduino Uno, a power shield (REf to model) and a bipolar stepper motor.

I cannot initiate the motor using the stepper library from Arduino. I instantiate my stepper with

Stepper myStepper(motorSteps, motorPin1,motorPin2m motorPin3,motorPin4);

And keep on with the example code provided with the Arduino.

When I launch the code on the Arduino, the motor emits some sounds, but it does not turn.

How can determine the right motor pins to use? On the power shield, which mode is to be used, PWM or PLL?