Hi,
Did you reference and register AjaxControlToolkit in your Assembly?
Example code in Master File:
<%@ Master Language="C#" AutoEventWireup="true" CodeBehind="MasterPage.master.cs" Inherits="abs.cbn.ui.SiteMaster" %>
<%@ Register Assembly="AjaxControlToolkit" Namespace="AjaxControlToolkit" TagPrefix="cc1" %>
Hope this could help...
Regards,