Two ways - you can give your menu a fixed width, or the div that is containing it a fixed width.
To set a width for the menu itself, in your menu.css:
ul#cssmw2 {
cursor: default;
list-style-type: none;
margin: 0 0 0 0;
padding: 0 0 0 0;
}
You can add a width to this, so:
ul#cssmw2 {
cursor: default;
list-style-type: none;
margin: 0 0 0 0;
padding: 0 0 0 0;
width:960px;
}
and adjust the width and see the results in the page. Start with a width deliberately smaller (so the menu 'collapses'), and increase it until your menu fits in. This is easiest in Dreamweaver in 'split' view, with your menu showing in design view, and the menu.css stylesheet showing in the top.
The other way is to add a fixed width to the div containing your menu or include.