That function is modifying the math behind the Transform. The problem is that you're calling it directly from the class, not from an instance of the class. So you need the Transform component of your object, and you Unity being a good guy you can do that easily :

function Update(){
 if( Input.GetAxis("Horizontal") ){
  transform.Rotate(Vector3(0,0,1));
 } 
}

Not that different isn't it ? the important part is transform instead of Transform. If the object still don't rotate correctly, check your axis, or change the z on the rotation.