I'll record a small tutorial for you tomorrow afternoon.
/edit- just opened up my laptop real quick. If you follow the pencilhead[ish] tutorial you will actually notice me addressing how to turn a black and white image into a alpha which can then be used as a trackmatte. I've included the link to the tut along with time-code below. While it's obviously not exactly the same the idea of what I'm doing in there with "channel combiner" is essentially what you're going for. http://youtu.be/EWGmB1PG1hM?hd=1&t=3m53s
I know for a fact that Rozard went about it differently as there are numerous ways of going about this. The method that I implied in the tutorial is a little more versatile though.
If you are still having issues tomorrow I'll be more than happy to record a specific tutorial regardless though.