How to assign code to buttons?

the current code I have on the timeline now (where the buttons reside) is:

stop();

function playLifeboy(event:MouseEvent):void

{

gotoAndStop(3);

}

function playKrave(event:MouseEvent):void

{

gotoAndStop(2);

}

lifeboy_btn.addEventListener(MouseEvent.CLICK, playLifeboy);

krave_btn.addEventListener(MouseEvent.CLICK, playKrave);

These work fine the first time but when I return to them they don't work. (I was told I have to assign the code to the buttons, But how?)