A quick fix for those of us who have encountered the top admin menu in Joomla becoming unresponsive. You know, you click, but no sub menu appears? If you check your dev tool of choice (ie. Firebug), you'll most likely see something like this:
$ES is not defined
The issue has to do with mootools.js and how older versions of Joomla handle the rendering of the admin menus. Recently, I ran into this issue when I installed a newer version of mootools so that I could get a lightbox effect on a Joomla Virtuemart store. What I didn't realize at the time was this resulted in the admin menu failing.
The fix, thankfully, is quite easy.
The file is: /administrator/templates/khepri/js/menu.js
The changes are:
var elements = $ES('li', el);
var elements = $(adminMenu).getElements('li');
nested = $E('ul', element);
nested = element.getElement('ul');