OnMouseDown and GUI function

I'm trying to enable a level change when selecting a game object. I'm having a rough time and keep on getting "You can only call GUI functions from within ONGUI". Any help would be greatly appreciated. Here's my script so far:

 #pragma strict
 
 function Start () {
 
 }
 
 function Update () {
     if (Input.GetKey("mouse 0")){
         buttonPressed = true;
     }    
 }
 
 var buttonPressed = false;
 
 function OnGUI () {
     if (buttonPressed == true){
     // Make a background box
     GUI.Box (Rect (10,10,100,90), "Navigation");
 
     // Make the first button. If it is pressed, Application.Loadlevel (1) will be executed
     if (GUI.Button (Rect (20,40,80,20), "1st Floor")) {
         Application.LoadLevel ("first_floor");
     }
 }
 }