System.Security.Cryptography.Xml

Question

I was looking at cryptography examples on the msdn website and saw a sample using the namespace System.Security.Cryptography.Xml
When I tried using that namespace in Visual Studio 2005 (with .NET framework 2.0 installed), intelliSense did not recognize it and the compiler told me such a namespace doesn't exist. I thought maybe its part of .net 3.0, so I went to this link and downloaded all the suggested updates:
http://www.netfx3.com/blogs/news_and_announcements/archive/2006/11/06/.NET-Framework-3.0-has-been-released_2100_.aspx 
Yet this didn't solve my problem (same result as before). Please advise what to do.
-Andrew