.slideDown Help

.slideDown Help

<style type="text/css">
body {
    margin: 0;
}

ul {
    list-style-type: none;
    margin: 0;
    padding: 0;
    width: 25%;
    background-color: #f1f1f1;
    position: fixed;
    height: 100%;
    overflow: auto;
}

li {
    display: block;
    color: #000;
    padding: 0px 0 0px 0px;
    text-decoration: none;
}
.submenu1 {
display: none;
}

.submenu2 {
display: none;
}

.submenu3 {
display: none;
}

.submenu4 {
display: none;
}
</style>

<script>

</script>
 </head>
<body>

<ul id="Menu">
  <li><img src="../images/untitled-2.png" width="124px" height="100px" alt="Logo" /></li>
  <li><a class="active" href="#home">Home</a></li>
  <li id="mc1">Managed IT Services</li>
   <li id="submenu1" class="submenu1">Strategic Guidance</li>
   <li class="submenu1">Expert Technicians</li>
   <li class="submenu1">Full Service</li>
   <li class="submenu1">Help Desk</li>
   <li class="submenu1">Same Day Service</li>
  <li id="mc2">IT Solutions</li>
   <li class="submenu2">Business Continuity</li>
   <li class="submenu2">Network Security</li>
   <li class="submenu2">Security Systems</li>
   <li class="submenu2">Cloud Services</li>
   <li class="submenu2">Data Backup/Transfer/Restore</li>
   <li class="submenu2">website Solutions</li>
   <li class="submenu2">VOIP</li>
   <li class="submenu2">Financing</li>
   <li class="submenu2">Same Day Service</li>
  <li id="mc3">About</li>
   <li class="submenu3">Custom Builds</li>
   <li class="submenu3">Access Control Servers</li>
   <li class="submenu3">Imaging</li>
  <li id="mc4">About</li>
   <li class="submenu4">Business Technology Consulting</li>
   <li class="submenu4">HW & SW</li>
   <li class="submenu4">Deployment & Data Transfer</li>
   <li class="submenu4">Network Design</li>
   <li class="submenu4">Network Assessment</li>
   <li class="submenu4">Financing</li>
</ul>

<script>
window.onload = function() {
 $("#mc1" ).mouseenter(function() {
$("#submenu1").slideDown( 5 );
 });
};
</script>



Above is the code for part of my website. I am trying to understand Jquery and I can't seem to get this slideDown script to work. What am I doing wrong?