http://msdn.microsoft.com/en-us/library/cc506546.aspx
The namespace is in .NET Micro Framework. Thus, you cannot use it in WinForms applications.
To connect to netduino and manipulate its hardware, your WinForms app should use a communication channel opened to the app running on netduino, such as via USB,
http://blog.mark-stevens.co.uk/2011/05/windows-application-communicating-with-netduino-over-usb/