It sounds like you need to wrap the menu inside a div which is nested inside the horizontal div and then style the CSS to get what you need, like this:
(you menu include here)
Then make sure in your CSS, that menuWrapper is set to go the full width you want it to, such as:
Or, input the value of the entire width of your page... and then to center the menu, style the margins on the inside div like this:
That should take your entire menu and put in the center.