Update EmptyGameObject Rotation to MousePosition

Why doesn't this seem to work?

function Update (){
if(Input.GetButtonDown("Fire1")){
transform.LookAt(Input.mousePosition);
}
}

This script is attached to the PrefabSpawnPoint I would like to rotate toward where I click on the screen.

This seems to be the simplest solution, but not working.

Input.mousePosition is not a place in the world; it's a place on your screen.

http://unity3d.com/support/documentation/ScriptReference/Camera.ScreenToWorldPoint.html