Won't let me add a layer mask...

...I'm not entirely sure what's going on because I've always been able to add layer masks before. I'm hoping someone will know what's happening.

This is what I'm doing: I open two pictures that I've saved and move one image on top of the other. Once I have the image placed where I want it to be, I go up to layer>add layer mask>reveal all. I use my paint brush tool, select what size I want and paint over the top layer but nothing is happening. And I don't understand because it's worked before and now it isn't. Maybe I'm doing something wrong--I don't know. I have Adobe Photoshop 8.0.

Does anyone know what's happening???

do you have black and white as your colors in the top and background colors of the tool bar. and is the proper color selected?

Ahh...now I see...I had black and white selected but I did not have the right color chosen. Thank you very much for your help.