Drop Down Menu Not working

Hi I am trying to add a drop down menu to my site but the drop down thing does not appear. Following is the code that I am using. If there is an error let me know.
Thank You

<ul id="nav">
<?php wp_nav_menu( array( 'theme_location' => 'header-menu', 'container' => '', 'menu_class' => 'nav', 'menu' => '' ) ); ?>
</ul>

CSS

#nav {
-moz-background-clip:border;
-moz-background-inline-policy:continuous;
-moz-background-origin:padding;
z-index: 9999;
width: 980px;
height: 34px;
background: url('images/menu-bg-red.png') no-repeat;
float: left;
list-style: none;
position: absolute;
top: 72px;
}
#nav, #nav ul {
line-height:40px;
list-style-image:none;
list-style-position:outside;
list-style-type:none;
width:100%;
}
#nav a, #nav a:hover {
-moz-background-clip:border;
-moz-background-inline-policy:continuous;
-moz-background-origin:padding;
font-family: 'AmericanTypewriterBold';
font-size: 15px;
border:medium none;
display:block;
text-decoration:none;
}
#nav li {
	display:inline;
padding: 0px 10px 4px 9px;
line-height: 25px;
float: left;
position: relative;
background: url('images/menusepperator.png') right 8px no-repeat;
}
#nav a:visited {
color:#FFFFFF;
display:block;
padding:0 0px;
}
#nav a:hover, #nav a:active, #home .on {
text-decoration:none;
color:#000000;
}
.current_page_item a{
	color:#ffffff;
}
#nav li ul {
height:auto;
left:-999em;
line-height:15px;
margin:0;
padding:0;
position:absolute;
background: url('images/dropDownbg.png') center bottom repeat-x scroll;
width:200px;
}
#nav li li {
padding: 0px;
float: none;
line-height: 15px;
margin: 10px;
background: none;
z-index: 10;
}
#nav li li a, #nav li li a:visited {
-moz-background-clip:border;
-moz-background-inline-policy:continuous;
-moz-background-origin:padding;
background:none repeat scroll 0 0;
color:#FFFFFF;
font-size:13px;
}
#nav li li a:hover, #nav li li a:active {
-moz-background-clip:border;
-moz-background-inline-policy:continuous;
-moz-background-origin:padding;
background:none repeat scroll 0 0;
color:#cccccc;
font-size: 13px;

}

#nav li:hover ul, #nav li li:hover ul, #nav li li li:hover ul, #nav li.sfhover ul, #nav li li.sfhover ul, #nav li li li.sfhover ul {
left:auto;
}