I found the culprit. In the menu there are floating elements, so a clearfix div was necessary.
In the attachment I have split the menu-menu.css in two, the global.css is the one where I have added the centering and the clearfix. In your original menu-menu.css I see that you have added styles for other elements than just the menu. I suggest that you transfer those styles to the global.css, so that you don't overwrite the menu-menu every time you change something.
I may not have hit the exact colours, sizes and widths of your menu since I didn't have your .mnu file. If you change the width, then remember to change the width of the .center class (in global.css) accordingly.
Another suggestion would be to line up the text and images on the first white pane, but that is a different subject...
