The mask should be in the top layer. Where there is black the bottom layer will show through, where there is white it won't. Everything in between has its own value. You can make straight blends, by using a gradient, or use any paint tool (and/or filter for that matter) in the mask. Oh. You can make a layer mask by selecting the layer and clicking the small square with the dotted circle, on the bottom of the 'layer' tab. Hope this helps...