I am new to AS3. I am creating a series of Flash Activities for an educational website and would like to put common function into their own class. I am not sure how to do that. What is have tried is:

1. Import the class( import MMESCommon
2. creating a new variable/object(var MMES:Object = new MMESCommon
3. Calling a function in the class(MMES.LetterScore(95)

This process( or one very similar) has worked for me in other languages. However, here I am getting the error:

ReferenceError: Error #1069: Property LetterScore not found on MMESCommon and there is no default value.
at AdditionLevel01/DrawGameFrame()
at AdditionLevel01/checkanswer()

I know LetterScore exists. here in the MMES Class:
Code:
/**
 * Mrs. Marker's Education Site Common Flash funcions
 * This file contains the action script for the MMES Common class.
 * AS3
 * MMES(tm) : Mrs. Marker's Educational Site (http://mrsmarker.dyndns.org)
 * Copyright 2012, Mrs. Marker's Educational Site (http://mrsmarker.dyndns.org)
 * @copyright     Mrs. Marker's Eduational Site (http://mrsmarker.dyndns.org)
 * @link          http://mrsmarker.dyndns.org MMES(tm)
 * @package       MMESCommon
 * @license       Propietary, All Rights Reserved
 */
package {
	/**
	 * Global import commands
	 */
	import flash.display.*;
	import flash.events.*;
	/**
	 * MMESCommon Class definition
	 * This class contains methods used by some/all of the MMES Flash Programs
	 * @extends MovieClip
	 */
	public class MMESCommon extends MovieClip {
		/**
		 * LetterScore function
		 * Funtion to take numberal score and return a letter grade.
		 * @param	score
		 * @return  String: The letter grade earned
		 */
		public function LetterScore(score:int):String {
			var retval:String = "";
			if (score >= 90 && score <= 100) {
				retval = "A";
			} else if (score >= 80 && score <= 89) {
				retval = "B";
			} else if (score >= 70 && score <= 79) {
				retval = "C";
			} else if (score >= 60 && score <= 69) {
				retval = "D";
			} else if (score >= 50 && score <= 59) {
				retval = "F";
			} else {
				retval = '--';
			}
			return retval;
		}
	}
}
What could I be doing wrong? Thanks for any help, Troy