If you create an adjustment layer, then there will be a layer mask to the right of the icon for the layer - it looks like a white rectangle. You have to click on that white rectangle to select the mask instead of selecting the layer itself. Does that make sense? Then you can paint with black to "erase" the layer mask's effect and paint with white to re-add it. Check out this screenshot I found on the web:
if you click on the box with the "T" then you're editing that text layer. If you click on the white box to the right of the "T" icon then you're editing the mask. I hope this helps.