Are you marking your response as correct because you solved it?
Assuming you didn't, my guess is that you want to have the lines where you add the ENTER_FRAME lsistener and start the timer to be executed by some control in your menu. As you have them now they will start immediately. Place them inside a function and have a button in your menu call them... something similar to the following would be a starting point...
startPlayingBtn.addEventListener(MouseEvent.CLICK, startPlaying)
function startPlaying(evt:MOuseEvent):void {
addEventListener(Event.ENTER_FRAME,OnEnter)
myTimer.start();
}