You have to edit inc/functions_forumlist.php first. Otherwise Plugin will not work!
Find:
           eval("\$forum_list .= \"".$templates->get("forumbit_depth$depth$forumcat")."\";");
 replace with:
	   if(!in_array($forum['fid'], explode(',', $mybb->settings['hideonindex_forums']))) {
	   eval("\$forum_list .= \"".$templates->get("forumbit_depth$depth$forumcat")."\";");
	   }
