To make the luck stat visible is actually pretty easy, but you will need to place it yourself.
Basically, to display the stat in a window class you will need to use this code:
self.contents.draw_text (x, y, width, height, actor.luck.to_s)
where x is the x position, y is the y position, width is how much room you have in the horizontal direction (if the text is wider than this it will be shrunk), height is the room you have in the y direction (the text is centred between y and y + height), and actor is a Game_Actor object. If you go into Window_Status (which is where I assume you intend to place the stat), then you will probably notice that there is already an actor object there for you to use.