What's your VS version? 2005 or 2008? In VS 2005, you should refer the
System.Web.extensions.dll & also register in your page like below,
<%@ Register Assembly="System.Web.Extensions" Namespace="System.Web.UI" TagPrefix="asp" %>
So verify these things are there, being things are present & it still gives you error then mention more details. Let us know.
Verify this in your web.config file
<pages>
<controls>
<add tagPrefix="asp" namespace="System.Web.UI" assembly="System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"/>
</controls>
</pages>