Peter,
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:
<div id="menuWrapper">
<div id="menuFocus">
(you menu include here)
</div>
</div>
Then make sure in your CSS, that menuWrapper is set to go the full width you want it to, such as:
#menuWrapper {
width: 100%;
}
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:
#menuFocus {
margin-left: auto;
margin-right: auto;
}
That should take your entire menu and put in the center.
Cheers,
Brian