How to sub menu in the main menu using the css file

* MENU */

#nav {
 background: #e5e5e5;
 float: left;
 margin: 0; padding: 0;
 border: 1px solid white;
 border-bottom: none;
}

#nav li a, #nav li {
 float: left;
}

#nav li {
 list-style: none;
 position: relative;
}

#nav li a {
 padding: 1em 2em;
 text-decoration: none;
 color: white;
 background: #292929;
 background: -moz-linear-gradient(top, black, #3c3c3c 1px, #292929 25px);
 background: -webkit-gradient(linear, left top, left 25, from(black), color-stop(4%, #3c3c3c), to(#292929));
 border-right: 1px solid #3c3c3c;
 border-left: 1px solid #292929;
 border-bottom: 1px solid #232323;
 border-top: 1px solid #545454;
}

#nav li a:hover {
 background: #2a0d65;
 background: -moz-linear-gradient(top, #11032e, #2a0d65);
 background: -webkit-gradient(linear, left top, left bottom, from(#11032e), to(#2a0d65));
}


/* Submenu */

.hasChildren {
    position: absolute;
    width: 5px; height: 5px;
    background: black;
    right : 0;
    bottom: 0;
}

#nav li ul {
 display: none;
 position: absolute;
 left: 0;
 top: 100%;
 padding: 0; margin: 0;
}

#nav li:hover > ul {
 display: block;
}

#nav li ul li, #nav li ul li a {
 float: none;
}

#nav li ul li {
 _display: inline; /* for IE6 */
}

#nav li ul li a {
 width: 150px;
 display: block;
}

/* SUBSUB Menu */

#nav li ul li ul {
 display: none;
}

#nav li ul li:hover ul {
 left: 100%;
 top: 0;
}


#nav li ul



<ul id="nav">
	<li><a href="#">Item 1</a></li>
	<li><a href="#">Item 2</a>
            <ul>
                <li><a href="#">Sub 1</a></li>
                <li><a href="#">Sub 2</a></li>
                <li><a href="#">Sub 3</a></li>
            </ul>
        </li>	
	<li><a href="#">Item 3</a></li>
</ul>

<link href="/css_menu.css" type="text/css" rel="Stylesheet"/>
                <div id="menuwrapper">
                        <ul>
                            <li><a href="http://xxx.com/contact.php"">Contact</a></li>
                            <li><a href="http://xxx.com/product_template_list.php">Produc Lines</a>
//SUBMENU OF PRODUCT LINE
                                <ul>
                                    <li><a href="#">Product 1</a></li>
                                    <li><a href="#">Product 2</a></li>
                                    <li><a href="#">Product 3</a></li>
                                </ul>
                            </li>
                            
                            <li ><a href="http://xxx.com">Home</a></li>
                         </ul>
                    </div>

/**** SECOND LEVEL MENU ****/
/* We make the position to absolute for flyout menu and hidden the ul until the user hover the parent li item */
#menuwrapper ul li ul{
    position:absolute;
    display:none;
}

/* When user has hovered the li item, we show the ul list by applying display:block, note: 25px is the menu height.  */
#menuwrapper ul li:hover ul{
    left:0;
    top:25px;
    display:block;
}

/* As the parent li has float:left property, we overwrite for the submenu (level 2) to float none (normal position)  */
#menuwrapper ul li ul li{
    float:none;
    width:150px;
    background-color:#cae25a;
}

/* We change the background color for the level 2 submenu when hovering the menu */
#menuwrapper ul li:hover ul li:hover{
    background-color:#b1b536;
}

/* We style the color of level 2 links */
#menuwrapper ul li ul li a{
    color:#454444;
    display:inline-block;
    width:120px;
}