Here's an item on MathWorks File Exchange -- you'll find a package with a guide that explains, in a step-by-step fashion, how to develop device driver blocks (blocks that perform target-specific functions when executed on a target platform). There are examples for a DC motor and a stepper motor, both with the Arduino AF Motor Shield.
General info re. using Arduino with MATLAB and Simulink is at Arduino Programming with MATLAB and Simulink - MATLAB & Simulink