henry,
Unlike Java, C# and other .net languages are compiled to an executable. The class files associated with the project are managed by the visual studio IDE. With this in mind, you don't need to specify a class path. Instead, if you are using folders within your project, you need to specify the folder using the . operator.
Example
WindowsApplication1
-GUIFolder
-MyGUIControl
-References
-Form1
For form 1 to access MyGUIControl, it would need to say:
WindowsApplication1.GUIFolder.MyGUIControl guiControl = new WindowsApplication1.GUIFolder.MyGUIControl();
To shorten this process, .net allows you to specify paths with the "using" keyword.
using WindowsApplication1.GUIFolder;
Form1 can then say MyGUIControl guiControl = new GUIControl();
In short form: .net is magic.
Hope that helps in some way