i'm not sure how you have the file set up, but based on what you said my understanding of it is this:

you want your movie to play automatically and stop when it reaches the end. then, you want the option to replay?

if so, go your last frame add stop(); to the actions panel then follow my previous instructions:
* insert new layer
* on last frame insert keyframe
* add button to key frame
* give button instance name & add button code to actions panel

the other scenario i see is you want it to wait for a click to activate.

if so:

* insert new layer
* add button & code to first frame
* select all frames and drag them over 1 frame
* put stop(); into the actions panel on frame 1 - so now your first animation frame and button are on frame 2
* make the button code point to frame 2 instead of 1. - i avoid using play() because with some of my past experiences triggering actions really fast confused flash and it screwed up the timing.