Help with buttons and child mc

Hello guys, first of all sorry for the poor english.
I’m kind of newbie in AS3 (how I miss AS2) and I’m going crazy with this simple thing:
I followed tutsplus.com tutorial and built my fluid layout:

package {


    import flash.display.*;
    import FluidLayout.*;
    
    public class Website extends MovieClip{
        
        public function Website()
        {
            /* Set the Scale Mode of the Stage */
            stage.scaleMode = StageScaleMode.NO_SCALE;
            stage.align = StageAlign.TOP_LEFT;
            
            /* Add the symbols to stage */
    
        
            var menu = new Menu();
            addChild(menu);
        
            var middle = new Middle();
            addChild(middle);
            
            /* Apply the alignment to the menu */
            var menuParam = {
                x:1,
                y:0,
                offsetX: -menu.width - 20,
                offsetY: 20
            }
            new FluidObject(menu,menuParam);
        
            /* Apply the alignment to the content */
            var middleParam = {
                x:0.5,
                y:0.5,
                offsetX: -middle.width/2,
                offsetY: -middle.height/2
            }
            new FluidObject(middle,middleParam);
        

The question is: In the MC MENU I have a button called “Portfolio”. What I want is that this button makes MIDDLE go to a specific frame, how can I do it? The code must be placed in the main timeline or in the MENU MC timeline?

There are others MC’s in this movie… In AS2 I used to do that simply using “.root.middle.gotoandplay()” but with AS3 and this fluid layout, I’m totally lost.

Can you guys help me?