wordpress - center menu

Hi Guys!

I'm stuck with the site I'm currently working on. Here is the link.

Here is what I want:
The menu should center horizontally on top of the page.
It also should be flexible, so i don't have rewrite the css if I'm adding a new menu item. The best would be if this worked automatically.

I'd be very thankful if someone could have a quick look at the page and could tell me what to do, or what I do wrong.

I tried a whole bunch of different tutorials and like a ton of different codes. But it just wont work. I'm no pro so I'm working mostly on trial and error. And this just comes out as error

THANK YOU,
colophonius