i imagine that your code of:

this.movieClip.play_btn.onRelease = function() {
gotoAndStop(5)
};

is nested inside some other object creating function - if so you may want to post the entire function.

if not try this:

movieClip.play_btn.onRelease = function() {
movieClip.gotoAndStop(5);
};

i have a feeling it is a targeting issue.. post your fla if want