Got stuck in inpainting, dont know how to relace an object with another one, help please

Hi Im so excited that I can use your plugin in photoshop, but as I want to replace some objects like the pagoda and trees with nothing but fog and mist over the water, for some reason I just can't manage to do it. I drew a rectangle, hit "Generate Inpaint", and it either gave me an blank white or black image, could you expain how I should use the inpaint function? Thanks!