If you want to add the page class to the body, then add this code above the doctype:
<?php
$app = JFactory::getApplication();
$menu = $app->getMenu()->getActive();
$pageclass = '';
if (is_object($menu))
$pageclass = $menu->params->get('pageclass_sfx');
?>
And this code to your body
<body id="<?php echo $pageclass ? htmlspecialchars($pageclass) : 'default'; ?>">
Anything without a page class will use an id of "default", anything with the page class defined will appear as the id of the body. Obviously you can change that id to class if that's what you need.
<div>
within the main content area.