Throw new error/ error creating control

Hello everyone!
Please help,
I am trying to add AJAX tabs to my asp.net website.  I am an absolute begineer too.
I followed one the Microsoft tutorials and I had the tabs working fine at university with this code:
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<%@ Page Language="VB" AutoEventWireup="false" CodeFile="Tab.aspx.vb" Inherits="Tab" %>

<%@ Register Assembly="AjaxControlToolkit" Namespace="AjaxControlToolkit" TagPrefix="cc1" %>

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">

<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
    <title>Tab Example</title>
   
</head>
<body>
    <form ...