Sub menus doesn't appear in IE6

I got a problem with nice menu on my website. The submenus doesn't appear in IE6, but it works well in other browsers. I've been trying to use methods from previous issues, but none of them is working for me.

I've tried to use position:relative, display:block, overflow:visible, none is working. I'm not sure whether it's the theme-layout CSS problem or the styling of the nice menus.

Here's the link to the website : http://www.sicog2009.org/

Maybe someone can help me with this issue? I really need help here. Thank you very much!