To use a c# class in labview call the constructor node from the labview .net tools, this will return a handle to the object created this handle once wired to a property node or method node will give you access to all (public?) methods and poperties of that particular class, unlike dlls all method signatures seem to be automatically known. I have been able to easily invoke c# forms and manipulate them in labview but have not seen a great need you to use the c# classes available from FCL provided from Microsoft. I am sure this will change as .net gains popularity.
-Paul