You need to add AjaxControlToolkit dll reference in your project and also register the ajaxtoolkit in the page like below.
<%@ Register Assembly="AjaxControlToolkit" Namespace="AjaxControlToolkit" TagPrefix="cc1" %>
Then you can use it like below.
<cc1:ToolkitScriptManager ID="ToolkitScriptManager1" runat="server">
</cc1:ToolkitScriptManager>