I did struggle in this lesson to keep up with the instructions but I eventually caught up an understood where to place the nodes to create the effect I wanted.
Using the transform and invert nodes we were able to slide the doors open, showing the matte painting behind. We adjusted the colour saturation in the alpha channels so that the doors layer matched the matte painting behind.
The next stage is to add the 'green screen' video behind the opening doors. Once this is done, it will look like the man is stepping through the sliding doors.