Attaching traveling matte URGENT

I have a very cool animation of a bank vault door opening that has a traveling matte as part of the end of the clip (BTW the animation camera view is moving along the Z axis into/through the opening door - the BW matte does the same and should allow me to key through this to reveal the contents inside the vault - at least that's what I am trying to do!)

Doing this on an Editbox was a snap - just drop the matte onto the foreground video - but I can't seem to get Motion to do this at all. I see where I can add an image mask to the video and then the HUD gives me the "image well", but I can't drag the Quicktime clip of the matte into the well - it won't go.

I'll keep plugging away but if anyone knows how to do this I'd appreciate the help.

Motion 3.0.4 and OS X 10.5.4 on a Macbook Intel 2.0 Dual Core

drag the matte into the layers tab first, then add it to the image mask.