the code...

YOUR INSTANCE BUTTON NAME.buttonMode = true;


YOUR INSTANCE BUTTON NAME.addEventListener(MouseEvent.ROLL_OVER,animIn);
YOUR INSTANCE BUTTON NAME.addEventListener(MouseEvent.ROLL_OUT,animOut);


function animIn(event:MouseEvent):void {
event.target.gotoAndPlay("over");
}
function animOut(event:MouseEvent):void{
event.target.gotoAndPlay("out");
}