problem in using a Tree and implementing a CompareTo() method inherrited from system.Icomparable

I'm a beginner in C# programing. I want to make and use a Tree in this project but I don't know how to implement CompareTo() method in my tree class.
You can download my Solution from link below
http://soroushweb.persiangig.com/BinaryTree.zip
I get the error below
thank you
Error    1    'BinaryTree.Tree<TItem>' does not contain a definition for 'CompareTo' and no extension method 'CompareTo' accepting a first argument of type 'BinaryTree.Tree<TItem>' could be found (are you missing a using directive or an assembly reference?)    C:\Documents and Settings\Mahdi Jafari\My Documents\Visual Studio 2008\Projects\BinaryTree\BinaryTree\Tree.cs    23    22    BinaryTree