First of all, you should add a RigidBody component to your GameObject. After that, you need to refer to that component, otherwise the game will not know, which RigidBody should get the force you are adding.

 var rotationSpeed = 100;
 var rb: RigidBody;
 
 function Start()
 {
     rb = GetComponent(RigidBody) as RigidBody;
 }
 
 function Update()
 {
     var rotation : float = Input.GetAxis ("Horizontal") * rotationSpeed * Time.deltaTime; 
     rb.AddRelativeTorque (Vector3.back * rotation);
 }