I had the same problem.... if you have changed the header.tpl file for customizing... I think you have forgotton to add the reference to Javascripts.

Check if the code between the <head></head> tag is same as below in header.tpl.

Code:
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<title>{$site_name} - {if $html_title ne ""}{$html_title}{else}Share Your Videos{/if}</title>
{if $html_keywords ne ""}<meta name="keywords" content="{$html_keywords}">{/if}
{if $html_description ne ""}<meta name="description" content="{$html_description}">{/if}
<link rel="alternate" type="application/rss+xml" title="20 Latest videos" href="{$baseurl}/rss/new/">
<link rel="alternate" type="application/rss+xml" title="20 Most Viewed Videos" href="{$baseurl}/rss/views/">
<link rel="alternate" type="application/rss+xml" title="20 Most Commented Videos" href="{$baseurl}/rss/comments/">

<script language="JavaScript" type="text/javascript">
var imgurl='{$baseurl}/templates/images';
var baseurl='{$baseurl}';
</script>

{if $yui_disable eq ''}
<script language="JavaScript" type="text/javascript" src="{$baseurl}/js/yahoo-dom-event.js"></script>
<script language="JavaScript" type="text/javascript" src="{$baseurl}/js/connection-min.js"></script>
{/if}
<script language="JavaScript" type="text/javascript" src="{$baseurl}/js/common.js"></script>
<script language="JavaScript" type="text/javascript" src="{$baseurl}/js/myjavascriptajax.js"></script>
<link href="{$baseurl}/templates/css/style.css" rel="stylesheet" type="text/css">
<link href="{$baseurl}/templates/css/rating.css" rel="stylesheet" type="text/css">
</head>
Dave