Error message "variable not assigned" ?

I do not receive this error message if the GameObject is Static but how do you assign a variable that is Not Static or is a moving GameObject? I'm trying to avoid using a 'Find" script.

here's what the message says.

"UnassignedReference Exception: The variable myGUIPlane of 'GUIPlaneOn' has not been assigned. You probably need to assign the myGUIPlane variable of the GUIPlaneOn script in the inspector.

So I dragged and dropped the 'GUIPlane' gameObject into the variable slot in the inspector(little icon appears) but then I get an error message. What am I doing wrong? I thought that by dragging and dropping the gameObject into the variable slot that it is "assigned" creating a reference to that object. But it's not working with moving objects that can't be Static in my game.. What am I doing wrong? Any suggestions? Thanx

Here's 2 scripts. The 'Find' script works with no error messages. The other does not.

this script works

 function OnTriggerEnter( myzTrigger : Collider){
   if(myzTrigger.gameObject.name == "tum1-1"){
          var myGUIPlane : MeshRenderer =gameObject.Find("GUIPlane").GetComponent(MeshRenderer);  
         myGUIPlane.enabled = true;
                 
     }

}

This script gives me the error;

 var myGUIPlane : MeshRenderer;  
 function OnTriggerEnter( myzTrigger : Collider){
       if(myzTrigger.gameObject.name == "tum1-1"){
                  myGUIPlane.enabled = true;
                     
         }
 }    

I'd like to use the second script. Any suggestions? thanx