member login popup box not appearing

on my site when i click on the member login link at the top the popup box is not appearing and testing through firefox it doesn't return any javascript error.

Here's my site:

http://dev.uba-solutions.com/ourlabschool/labschool/

whatever you were working on do you remember a <div class="clear_both"></div> being around there? with one missing that will break....

well I've seen this code "<div class="clear_both"></div>"in quite a few place, can I know exactly where should be checking that??