help with mx transition

[macuser]

I have to make a small flash site and I want to use mx.transitions. Tween for smooth animation between info/images.
The problem is that when I push the button to finish the transition I don't know how to advance to the next frame.

The code is on the first frame - when the object enters the stage:
(z1 is the instance of the mc)

new mx.transitions.Tween(z1, "_x", mx.transitions.easing.Elastic.easeOut, 0, 300, 3, true);

The code on the button - when the object exists the stage:

on (press) {
new mx.transitions.Tween(z1, "_x", mx.transitions.easing.Elastic.easeOut, 300, 600, 3, true);
}

I know that exists the event handler .onMotionFinished ,but I don't know how to use to advance to next frame in the movie.

Thanks