Mattes in Red

I am just beginning to learn Red, and getting a little hung up on mattes.

If I want to matte a layer, I can add a still image track into the mask of the face. So far I can only get a matte to happen if the image I drop in has an alpha, and the alpha is used rather than the RGB value of the image.

If I have a B&W image or an animated B&W video or composite that I want to use as a mask, how do I make that work?

Ian