you can either add on the last frame of your timeline:
btn.btn_pause.visible = true;
or use a loop (like enterframe) that repeatedly checks the timeline's currentFrame and when it's == totalFrames, execute that statement:
this.addEventListener(Event.ENTER_FRAME,pauseF);
function pauseF(e:Event):void{
btn.btn_pause.visible = true;
}
the later solution is more complicated but has the virtue of keeping all your code on one spot.