Thanks for the update.

The hover effect is defined using the following classes (see Demo/Themes/themes.css):

.menu_main a span 
{
	display: block;
	height: 20px;
	line-height: 20px;
	width: 150px; 
	overflow:hidden;
	padding-left: 2px;
	padding-right: 20px;
}

.menu_main a:hover {
	background-image: url('menu_default/left.png');
	background-repeat: no-repeat;	
}

.menu_main a:hover span {
	background-image: url('menu_default/right.png');
	background-repeat: no-repeat;	
	background-position: 100%;
}

It looks like the IE6 mode isn't able to display it correctly. I tried a few quick tweaks but without much success. The demo layout is all broken in the IE6 mode so it's difficult to test it.

You may want to replace it with something more simple, like just adding text-decoration:underline on hover.