Modifying the Character Motor Script

 var walkSpeed: float = 7; // regular speed
 var crchSpeed: float = 3; // crouching speed
 var runSpeed: float = 20; // run speed
  
 private var chMotor: CharacterMotor;
 private var tr: Transform;
 private var dist: float; // distance to ground
  
 function Start(){
     chMotor = GetComponent(CharacterMotor);
     tr = transform;
     var ch:CharacterController = GetComponent(CharacterController);
     dist = ch.height/2; // calculate distance to ground
 }
  
 function Update(){
  
     var vScale = 1.0;
     var speed = walkSpeed;
  
     if (chMotor.grounded && Input.GetKey("left shift") || Input.GetKey("right shift")){
         speed = runSpeed;
     }
     if (Input.GetKey("c")){ // press C to crouch
         vScale = 0.5;
         speed = crchSpeed; // slow down when crouching
     }
     chMotor.movement.maxForwardSpeed = speed; // set max speed
     var ultScale = tr.localScale.y; // crouch/stand up smoothly 
     tr.localScale.y = Mathf.Lerp(tr.localScale.y, vScale, 5*Time.deltaTime);
     tr.position.y += dist * (tr.localScale.y-ultScale); // fix vertical position
 }

for some reason the website has put it all beside each other but just press enter after each of the semicolins to line it up and add that to ur character controller and you will now have a run and a crouch key