Ummm. Stepper motors don't move left or right. They rotate.
Can you read the position of the mouse? Can you tell when it is moving left or right, up or down?
If you can tell the motion/position of the mouse and you can drive the stepper motor, driving the stepper motor in response to the mouse movement should be easy.
Not trying to be rude, but you can't post a request for someone to write you code for a simple sketch and then expect them to do it in less than 2 hour besides. :