とりあえずこんな感じか。
(ちなみに C++ は main に限って return は不要。これ豆知識な)

int main(int argc, char** argv)
{
Gtk::Main kit(argc, argv);

AppWindow::Initialize(argc, argv);

Glib::RefPtr<Gtk::Builder> builder = Gtk::Builder::create_from_file(AppWindow::GLADE_FILE_NAME);

AppWindow* app_window = 0;
builder->get_widget_derived(AppWindow::WINDOW_NAME, app_window);

kit.run(*app_window);
}