Drop-down menus falling behind slider & page content

Word of warning: I am a WP novice so please forgive my stupid questions. 🙂

Site: http://www.grkroccenter.org
(http://grkroccenter.org/wp-content/themes/grkroc/style.css?ver=3.5.1)

Drop-down menus used to work fine until (I believe) we updated our jQuery about a month ago. Now the menus are dropping down behind the Nivo Slider and the main content of the pages.

I have done an exhaustive search for help, read lots about the z-index numbers, and have tried a few fixes but nothing has worked for me. I could easily be missing something obvious, or be putting code in the wrong place.

Help is much appreciated. Thanks in advance!