最後のliを選択した状態で表示しようとしていますが、最初はそのサブメニュー項目も表示する必要があります。
だからここに私が試したデモリンクがあります。
<!DOCTYPE html>
<html>
<head>
<meta http-equiv="Content-type" content="text/html; charset=utf-8">
<title>Test Page</title>
<link rel="stylesheet" href="styles/master.css" type="text/css" media="screen" title="no title" charset="utf-8">
<script src="js/jquery-1.3.2.min.js" type="text/javascript" charset="utf-8"></script>
<script src="js/modernizr-1.6.min.js" type="text/javascript" charset="utf-8"></script>
</head>
<body>
<div class="page">
<header>
<figure>
<img src="images/logo.png" width="48" height="49" alt="Logo">
<figcaption>
<h1>Company Name</h1>
</figcaption>
</figure>
<nav>
<ul>
<li><a href="home.html">Home</a></li>
<li><a href="aboutus.html">About Us</a></li>
<li class="submenuHeader">
<a href="products.html">Products</a>
<ul class="submenu">
<li><a href="webpage.html">Web Page</a></li>
<li><a href="mobileapp.html">Mobile App</a></li>
</ul>
</li>
</ul>
</nav>
</header>
<section>
<h1>Lorem Ipsum dolor</h1>
<h3>Lorem Ipsum dolor sit ame</h3>
<p>
Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat. Duis aute irure dolor in reprehenderit in voluptate velit esse cillum dolore eu fugiat nulla pariatur. Excepteur sint occaecat cupidatat non proident, sunt in culpa qui officia deserunt mollit anim id est laborum.
</p>
</section>
<form action="Test_submit" method="get" accept-charset="utf-8">
<h3>Registration Form</h3>
<label for="first_name">First Name</label><input type="text" name="first_name" value="" id="first_name">
<label for="Email">Email</label><input type="text/submit/hidden/button" name="" value="" id="">
<label for="country">Country</label><input type="text/submit/hidden/button" name="country" value="" id="country">
<p><input type="submit" value="Submit" class="button"></p>
</form>
</div>
</body>
</html>