Problem with styling links in navbar in Zen Theme

Hi,

I am working on a project for a NGO (ICNYP). The site can be viewed on http://www.einstern.eu

I would like to add some padding to the sublinks in the navbar (menu bar at the very top of the site). Click on "About US" and a link "Organs" appears. It is this link that I want to style.

Can you provide me with the right css tag?

Thanks
Chris