Wait for tween class ready (easy)

I have a menu, that I want to scroll according to mouse _y movement.
So I have the buttons in a mc, with the actions:

onClipEvent (enterFrame) {
	if (_parent._ymouse<149 && _parent._ymouse>7 && _parent._xmouse>13 && _parent._xmouse<138) {
		yPos = (-_parent._ymouse+35)/(this._height/165);

And in the timeline, at frame 1, the actions:

import mx.transitions.Tween;
import mx.transitions.easing.*;
var yPos:Tween = new Tween(buttons, "_y", Strong.easeOut, 0, 50, 2, true);
buttons.onEnterFrame = function() {
	yPos.continueTo(buttons.yPos, 0.7);

But it doesn’t work. I’t seems like the ‘buttons.onEnterFrame’ function has to wait for the new Tween to finish. How do I work around this?