I'm sorry, but I don't have the Adafruit Motor Shield (I gave it away for someone's birthday). So I can't test your code.
You should try to make smaller steps, and test the motor and button seperately.
Can you make to motor rotate, wait for a few seconds, and make it rotate in the other direction ?
I'm not sure about your switch, is it a push-button or a on/off switch ? Can you try it to make a led on and off, as if it was the motor or the direction ?