Make sure the Layer Mask is selected (the box on the right in the sky layer). You also want to make sure that you are working with black and white as the colors you're painting. Black will show the underlying layer (i.e. remove the curves adjustment you just did from your shot). To set your colors to black and white, type "d" (for default). You want to start with black on top. You can switch which color is on top by typing "x".

Make sure the Layer Mask is selected (the box on the right in the sky layer). You also want to make sure that you are working with black and white as the colors you're painting. Black will show the underlying layer (i.e. remove the curves adjustment you just did from your shot). To set your colors to black and white, type "d" (for default). You want to start with black on top. You can switch which color is on top by typing "x".