Moving a character with a RigidBody(javascript)

Hello, I am trying to make a character move with the w,a,s,d keys, and I was trying to do so with a RigidBody (3D, as this is a third person game). I came up with this code:

function Update () {
if (Input.GetKeyDown("w"))
(rigidbody.AddForce(0,0,5) );
}

But when I press W nothing happens. What am I doing wrong, and is there a better way to move a character?

Sample Assets have a very nice example of a RigidBody FPS controller. I suggest you to check it out.