//SideLinksDOM
      $SideLinksManager = new DataObjectManager(
         $this,
         'SideLinks',
         'SideLink',
         array(
             'Label' => 'Label'
         ), 'getCMSFields_forPopup'
      );
      
      $SideLinksManager->setParentClass('HomePage');
      $fields->addFieldToTab('Root.Content.SideLinks', $SideLinksManager);
      
      return $fields;

class SideLink extends DataObject{
   
   static $db = array(
      'Label' => 'Varchar(150)'
      
   );
   
   static $has_one = array(
      'Parent' => 'SiteTree',
      'Target' => 'SiteTree',
      'Image' => 'Image',
      'PageLink' => 'SiteTree'
      
   );
   
   public function getCMSFields(){
      
      $treeDropDownField = new TreeDropdownField("PageLinkID", "Choose a page to link to", "SiteTree");
      
      return new FieldSet(
         new TextField('Label', 'Label'),
         $treeDropDownField
      );
   }
}