Help with CharacterController.Move

I have this script here /// This script moves the character controller forward

/// and sideways based on the arrow keys.

/// It also jumps when pressing space.

/// Make sure to attach a character controller to the same game object.

/// It is recommended that you make only one call to Move or SimpleMove per frame.

var speed : float = 6.0;

var jumpSpeed : float = 8.0;

var gravity : float = 20.0;

private var moveDirection : Vector3 = Vector3.zero;

function Update() {

var controller : CharacterController = GetComponent(CharacterController);

if (controller.isGrounded) {

// We are grounded, so recalculate

// move direction directly from axes

moveDirection = Vector3(Input.GetAxis("Horizontal"), 0,

Input.GetAxis("Vertical"));

moveDirection = transform.TransformDirection(moveDirection);

moveDirection *= speed;

if (Input.GetButton ("Jump")) {

moveDirection.y = jumpSpeed;

}

}

// Apply gravity

moveDirection.y -= gravity * Time.deltaTime;

// Move the controller

controller.Move(moveDirection * Time.deltaTime);

}

i'm pretty sure its not the problem with my script becaus it was copied directly from the unity script reference. when i run, error pops up saying "MissingMethodException: Method Not Found: 'CharacterController.Move'." what can i do? i really want to know if anyone else is experiencing this really, really weird problem...... or if anyone can help! any help would be appreciated!