The error means you din't registered AjaxControlToolKit assembly. Register the assembly after @Page directive. Try this:
<%@ Register Assembly="AjaxControlToolkit" Namespace="AjaxControlToolkit" TagPrefix="asp" %>
And also add a reference of AjaxControlToolkit ddl in your solution. (Right Click on solution explorer --> Add Reference --> Browse Tab --> Browse the file then click on OK)
--Amit