Cant get video streaming texture to mask out?

Hi, I am trying to add smoke elements onto planes to add a smoke effect. The smoke element is a standard mp4 from action essentials library. The smoke itself doesnt have a alpha channel as its an mp4 and it cant store an alpha channel. However, as the elemtn is smoke, the rgb of the smoke is white anyway as the smoke is white on black, i would have though i could use that as the alpha. However no matter what i do i cant seem to get it to work. What am i doing wrong?