I\'ve referenced System.Xml:
using System.Xml;
Then in this line:
XmlDocument xdoc = new XmlDocument();
I
Make sure your project references the System.Xml.dll assembly. It's possible that you're referencing a different assembly that contains other System.Xml.* classes and that might be why you're seeing the System.Xml namespace but without the classes you need.
If you already have this reference, try removing and re-adding it and see if that irons out some weird VS glitch.