Which Framework you are using if its 3.5 then you have to download AjaxControlToolkit 3.5 version and if framework is 4.0 the download AjaxControlToolkit 4.0
AjaxControlToolkit is available here
http://ajaxcontroltoolkit.codeplex.com/releases/view/112805
After downloading create a Bin folder in your project and then Extract the downloaded file. you have to copy AjaxControlToolkit.dll and AjaxMin.dll into Bin folder
and at page you have register the Ajax as
<%@ Register Assembly="AjaxControlToolkit" Namespace="AjaxControlToolkit" TagPrefix="cc1" %>
And if you want to use Ajax say CalendarExtender then use like this:
<form id="form1" runat="server">
<cc1:ToolkitScriptManager runat="server">
</cc1:ToolkitScriptManager>
<asp:TextBox ID="txtDemo" runat="server" />
<cc1:CalendarExtender ID="ceDemo" runat="server" TargetControlID="txtDemo">
</cc1:CalendarExtender>
</form>
use ToolkitScriptManager instead of ScriptManager
