What is mod net?

For the problem there is many ways, but by fading you propably mean fading whole screen to black and back in to new screen.

In 2D mode you can accurately draw a faded black rectangle on top of other graphics, or you can use black glClearColor (is default) and change alpha color of all your graphics. Also to draw rect in solid color you need to disable texture.
Code:
tex.Disable;
  nx.Enable2D;
  nx.SetColor(0,0,0,fade);
  nx.RectT(0,0,nx.Width,nx.Height);
  nx.Disable2D;
  tex.Enable;