WindowsForms don't work in Unity. Instead we provide a very tightly integrated way for you to extend the unity editor. See Editor Windows in the documentation. For runtime, you can use the same api to get your runtime GUI up and running, GUILayout.* and friends.

I know this is kinda old but i just now saw it cause im just now using this site but anyways you can get a form up in unity using GTK in mono, just import the .dlls that you need into your project then create a form using the GTK, you can even use the visual editor in MonoDev. but combine the two .cs files that it makes for your window into one file then all you have to do is anywhere you want to start the form add this

    Gtk.Application.Init();
    MainWindow chatWindow = new MainWindow();
    chatWindow.Show();
    Gtk.Application.Run();

and then so it will close with the game add this into the function OnApplicationQuit()

    Gtk.Application.Quit();