kempachi

im using adobe flash CS4 and im making a tween effect with action script but the problem is when i test the movie it comes up with an error that says

The class or interface 'fl.transitions.Tween' could not be loaded.

here is my as for the flash video

import fl.transitions.Tween;
import fl.transitions.easing.*;

var startY:Number = welcome_MC._y;

operation_BTN.onRollOver = function () {
new Tween (welcome_MC, "_alpha", Regular.easeOut, 0, 100, 12, false);
new Tween (welcome_MC, "_y",Regular.easOut, startY, startY+30, 12, false);
}
if any one can help me it will be much apreciated