It's hard to tell what the problem is just looking at the code here, I would have to see it in context in your page.
As for the widths, you would need to specify a width in pixels.
the wrapping that you describe will occur in a liquid site. A liquid site will try to resize the menu div based on the size of the browser window, when the menu div shrinks, the menue will have to wrap onto another line.
Using a fixed width layout, this will not happen.