I have debugged link on your site and found the problem from SWmenuPro menu module from http://www.swmenupro.com, which makes all z-index style to z-index: 100. It’s from this script on your site
<script type="text/javascript">
<!--
if (jQuery.browser.msie && jQuery.browser.version < 9 ) {
jQuery('#outerwrap190').css('z-index','-1');
jQuery('#outerwrap190').wrap('<div></div>');
jQuery('#outerwrap190').parent().css('padding', '3px');
jQuery('#outerwrap190').parent().css('background-color', '#EFEFEF');
jQuery('#outerwrap190').corner('round tl tr bl br 6px');
jQuery('#outerwrap190').parent().corner('round tl tr bl br 9px');
}else{
jQuery('#outerwrap190').corner('keep round tl tr bl br 6px');
}
jQuery('#menu190').parents().css('overflow','visible');
jQuery('html').css('overflow','auto');
jQuery('#menu190').parents().css('z-index','100');
jQuery('#menu190').css('z-index','101');
//-->
</script>