Oct 11, 2013 03:53 PM|Dirceu Nascimento|LINK
You need to add the reference to System.Security.dll.
See http://msdn.microsoft.com/en-us/library/system.security.cryptography.xml.signedxml.aspx which shows that the assembly is System.Security.dll.
To add reference, click on the root in the Solution Explorer, and select >>Add Reference
Then on the tab .NET look up for System.Security.dll and click Ok
Then Rebuild the solution.