If you are driving a stepper motor then use a specialized stepper motor driver rather than an L298 which is really intended for DC motors. See Stepper Motor Basics
My guess (and it is no more than that) is that the driver in your link is intended to be controlled with a potentiometer rather than an Arduino.
Have a look at the Pololu website for a selection of DC motor and stepper motor drivers that work with an Arduino. Other suppliers will have similar devices.
...R