<?php

class Page extends SiteTree {
   static $db = array(
   );
   static $has_one = array(
);
}

class Page_Controller extends ContentController {
   function init() {
      parent::init();
      
      Requirements::themedCSS("layout");
      Requirements::themedCSS("typography");
      //Requirements::themedCSS("form");
   }
}

?>

Copy the main Page.php in mysite/code to a new file named with the same name as your theme Page.ss file...
MyPage.php
Then change the two lines of code
class MyPage extends SiteTree {
class MyPage_Controller extends ContentController {

I think there is some confusion. In the tutorial section there is no instruction to put the HomePage.php, the ArticlePage.php or the ArticleHolder.php files in the mysite/code directory. I had the same problem while going through the tutorial and after about 30 minutes came here to find an answer. I moved the php files to the mysite/code directory and then flushed the db and it worked fine. Is this an error in the tutorial?

I am so far pretty impressed with the extensibility of this cms.