Error 1010//Buttons Aren't Working

I know AS2 and am finally converting to AS3 now by following a book. My code is exactly the way it is in the book and everything in my library (the buttons, movieclips) are linked correctly, but I keep getting the same error 1010 that a term is undefined or has no properties. I want to know if I'm being dumb (I hope not), or if the book is outdated. Hopefully someone can help. I put in a picture of what my assets look like, as well as the code below it.

package  {

          import flash.display.MovieClip;

          import flash.events.MouseEvent;

          public class Main extends MovieClip{

                    var startPage:StartPage;

                    var hillPage:HillPage;

                    var pondPage:PondPage;

                    public function Main() {

                              startPage = new StartPage();

                              hillPage = new HillPage();

                              pondPage = new PondPage();

                              addChild(startPage);

                              startPage.HillButton.addEventListener(MouseEvent.CLICK, onHillButtonClick);

                              startPage.PondButton.addEventListener(MouseEvent.CLICK, onPondButtonClick);

                    }

                    function onHillButtonClick(event:MouseEvent):void{

                              addChild(hillPage);

                              removeChild(startPage);

                    }

                    function onPondButtonClick(event:MouseEvent):void{

                              addChild(pondPage);

                              removeChild(startPage);

                    }

          }

}