I have a video of a speaker at a forum. I have inserted his powerpoint slides on the top right of the window in the track above this (using pan and crop to size correctly). When the presenter moves behind a slide he is obscured by the slide, and I wish to make the slide transparent. I have consulted all web references, and have the book "The Movipix.com Guide to Sony Movie Studio Platinum 13" which explains WHAT to do, but not HOW to do it, by creating a rectangular mask using the Color Gradient transparent media generator using the transparent rectangular shape and inserting it on a new top track (created as a video track) in the same position as the slides and resized with the cropping tool. I have tried all the compositing and parenting mode combinations but cannot get it to work. Either one or all the tracks goes black, or the mask is the wrong size and only partially transparent. 

Can somebody please point me to an authoritative reference, tutorial, or video which will solve my problem?

Thanks folks

Stan