<script type="text/javascript">
        $(document).ready(function(){
            $(".sf-menu li").children("a").each(function() {
                if(window.location.href == $(this).attr('href')){
                    if($(this).parent().hasClass('main_link')){
                        $(this).addClass('active');                        
                    }else{
                        var topLink = $(this).parents('.main_link');
                        topLink.find('a').first().addClass('active')
                    }
                }
            });
        });
    </script>

look through the list items in your main nav and based on href of anchor tags this will set the top level link to have an 'active' class.

Set links with href to current page as active (or if in subnav set their parent to active)

css, javascript, navigation, jquery

JavaScript