Zen theme not showing second level menu items
Edit your page.tpl.php

Edit the section where the primary_links are printed to show the whole primary links menu instead of only showing the first level links of your primary links menu.

&lt;?php if (isset($primary_links)) { ?&gt;
     &lt;div id="primary_menu"&gt;
&lt;?php print theme('menu_tree',variable_get('menu_primary_menu',0)); ?&gt;
     &lt;/div&gt;
&lt;?php } ?&gt;


In Drupal 6

&lt;?php
$menu_name = variable_get('menu_primary_links_source', 'primary-links');
print menu_tree($menu_name);
?&gt;


Drupal 7

&lt;?php

    $menu_name = variable_get('menu_main_links_source', 'main-menu');
    $tree = menu_tree($menu_name);
    print drupal_render($tree); 
?&gt;