Keyboard class not working with CS4 AS3?

I am starting out using Action Script on flash cs4 and found out that whenever I try to use the Keyboard class e.g Keyboard.DOWN I always get "1120: Access of undefined property Keyboard."

Sorry if this has answered before but I cannot seems to find similar question. Here is a very simple AS3 I created trying to play around with the keyboard. Please do help me

package {

    import flash.display.*;

    import flash.events.*;

    public class Main extends MovieClip{

        public function Main(){

                stage.addEventListener(KeyboardEvent.KEY_DOWN, movements);

                function movements(event:KeyboardEvent){

                if(event.keyCode == Keyboard.LEFT);

                trace("success");

                }

        }

    }

    }