first of all you have to move your UL and its related li elements to be btween
<div> and </div> Tags, so they will be related to the div element, then you have several options to centering your UL menu, choose one of them.
1- using css code to DIV will center all text including your menu.
2- adding some padding space to DIV will move all div element to start after 70 pixel from the left.
3- moving the main DIV position to the place to want by add this code