Hi,
add this in style block in index.php of template just before </head>
Code:
<style type="text/css">
#topmenu_pad
{
width:1000px;/*You may need to adjust this value*/
}
</style>
also you should replace mega-menu core is not related to your problem but it good thing to do(in attachment).
templates\hot_wellness\js\jquery.hjt.megamenu.js