Not sure I quite understand what you’re trying to achieve. So I’ll stab at your question and probably leave it sore :-> My assumption is that you have a high-contrast (pure white, pure black) matte, and not a gradiated matte.
If you’re trying to use a matte to “knock out” sections of the still frame in PS, simply magic wand (select) the white area of the matte, highlight the video frame layer, and hit backspace or delete. If you’re trying to use the matte to key in your video editor, the process varies a bit. Sounds like you described an Avid matte, if so, place your video track on a layer. Place the matte in the layer above the video. Apply the “matte key” effect to the matte. Voila! Your black areas are transparent, your white areas keyed out.