There are 2 mouse look scripts that come with your first person controller. You would have to turn on/off both.

If you want to toggle mouse visibility and mouselook then try the following:

private var fpcMouse;
private var camMouse;

function Start () {

fpcMouse = gameObject.Find("First Person Controller").GetComponent("MouseLook");
camMouse = gameObject.Find("Main Camera").GetComponent("MouseLook");

}


function Update () {

if (Input.GetButtonDown ("Fire2")) {
    Screen.showCursor = !Screen.showCursor;
    fpcMouse.enabled = !fpcMouse.enabled;
    camMouse.enabled = !camMouse.enabled;
     }

}

basically you should add 2 statements to your script.

void Update ()
{
if (Input.GetButtonDown("some button here"))
{
Screen.showCursor=!Screen.showCursor; //if true it will become not(true)=false and if false it will become true
this.enabled=!Screen.showCursor; //this is a reference to current script. you should add this
//to mouse look script to work. if this is another script then you should use GetComponent    
}

you should use GetComponent like this

var ml=GetComponent("MouseLook");

then you should use ml.enabled rather than this.enabled. if the MouseLook is in another gameObject use what sebas provided with your GameObject name.

basically you should add 2 statements to your script.

void Update ()
{
if (Input.GetButtonDown("some button here"))
{
Screen.showCursor=!Screen.showCursor; //if true it will become not(true)=false and if false it will become true
this.enabled=!Screen.showCursor; //this is a reference to current script. you should add this
//to mouse look script to work. if this is another script then you should use GetComponent    
}

you should use GetComponent like this

var ml=GetComponent("MouseLook");

then you should use ml.enabled rather than this.enabled. if the MouseLook is in another gameObject use what sebas provided with your GameObject name.