(use "if" "cookie value= something" rule here instead of MosCountModules)
<?php if(!$editmode and (mosCountModules('right') or ($sidenav and $menu_side == 'right'))) : ?> 
     <td class="right">
                              
     <?php if($sidenav and $menu_side == 'right') : ?>
&nbsp; &nbsp; <div id="vert-menu">
<?php echo $sidenav; ?>
     </div>
      <?php endif; ?>
<div id="toggle2" >
</div>
<div id="test2">
<?php mosLoadModules('right', -2); ?>
</div>
</td>
<?php endif; ?>

<script type="text/javascript">
window.addEvent('domready', function(){ 
     //just an habit, not necessary but i make less mistakes this way than when using $ or $$ each time... always end forgetting a ' somewhere :)
     var toggle2 = $('toggle2');
 
     var mySlide2 = new Fx.Slide('test2', {
          mode: 'horizontal', duration: 500, //transition: Fx.Transitions.Bounce.easeOut,&nbsp; //i let you add the other options, let's just concentrate on that cookie issue, don't need mode: vertical, it's the default mode
          onComplete: function(){
               if(this.wrapper.offsetWidth != 0) {
                    Cookie.set('toggleState', 'opened',&nbsp; {path: "/"});
               }
               else {
                    Cookie.set('toggleState', 'closed', {path: "/"});
               }
          }
     })
     
     //here you check existence and value of 'toggleState' in a cookie
     if ( (Cookie.get('toggleState')) && (Cookie.get('toggleState') == 'opened') ) {
          mySlide2.show();
     }
     else {
          mySlide2.hide();
     }
     //here is you simple toggling link click event
     toggle2.addEvent('click', function(e){
          e = new Event(e).stop();
          mySlide2.toggle();
     });
 
});
     
</script>