Fix drop down menu

Hello,

My theme was not support drop down menus, I tried to fix it but still there is a problem: http://www.ieeeday.co.cc

Anybody can help??

Menu in header.php :

<!-- Site Navigation -->
   <div id="navbar">
      <div id="navigation">
          <!-- load widget for pages -->
          <?php wp_nav_menu( array( 'container_class' => 'menu-header', 'theme_location' => 'primary', 'depth' => 0 ) ); ?>
      </div>
   </div>

CSS:

/* Site Navigation Bar */
#navbar {
	padding: 0 20px;
}

#navbar #navigation {
	width: 944px;
	height: 39px;
	font-size: 1em;
	overflow: hidden;
	background: url(../images/bg_nav.png) no-repeat;
}

#navbar #navigation ul {
	margin: 0;
	padding: 0;
	width:auto;
	position:relative;
	display:block;
	height:39px;
}

#navbar #navigation ul li {
	display:block;
	float:left;
	margin:0;
	padding:0;
}

#navbar #navigation ul li a {
	display:block;
	float:left;
	text-decoration:none;
	font-weight:bold;
	height:26px;
}

#navbar #navigation ul li a.double {
	padding: 0;
	height: 31px;
}

#navbar #navigation ul li a:hover {
}

#navbar #navigation ul li a.last:hover  {

}

#navbar #navigation ul li a.active, #navbar #navigation ul li.current_page_item a, #navbar #navigation ul li.current_page_parent a {
	color: #ffffff;
	text-decoration: none;
	background-image: url(../images/bg_nav_on.png);
	background-position: 0 1px;
}

#navbar #navigation ul li ul {
	display: none;
}

#navbar #navigation ul li:hover > ul {
	display: block;
        float: left;
        z-index: 99999;
}