Hello Michael Chan Fong,
Thank you for using our services.
Since VS 2010 is still in beta version we do not officially support it and cannot guarantee that the Sitefinity website will compile in this environment.
This issue appears to be from the references that are to the System.Web.Extensions namespace. This is a namespace from .NET framework 1.0 and if you do not have it installed on the computer will get the error you were getting. Since you have said that you are with .NET 3.5 those registers are unnecessary. We have even excluded those from the control templates and those fixes will be available in the next SP release of Sitefinity 3.7.
so the fix for you will be to remove the following register tags from the files that you are getting the errors in:
<%@ Register Assembly="System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"
Namespace="System.Web.UI" TagPrefix="asp" %>
Those will be removed in the next SP of Sitefinity.
the Telerik team