How do you AddForce to a rigidbody2D in the direction of a joystick?

I've been trying to figure this out for a while now but I cant seem to get it right. I need to use two joysticks (the joysticks of an xbox controller) to AddForce in the direction they're facing. I set this up for the W, A, S, and D keys just to test if this is is what I want. This is the basics of what that script is:

 var movePart1 : Rigidbody2D;       // I need to move two things at once with the same
 var movePart2 : Rigidbody2D;       // joystick.
 
 function Update () {
 
 if (Input.GetButton("w")) {
 MoveFoot1.AddForce (Vector3.up * 7);
 }
 if (Input.GetButton("a")) {
 ETC...

So... How should I make this work with any angle / joystick input?