OK, outside the tutorial this is my first go at making something in Motion.
I have a couple of TIFF files and I want them to flip over to reveal a different image on the back. So I go into Filters and pick out "Basic 3D" which gives me a nice flip. However the "Back Face" picture well will not accept the image I am trying to put in it. Actually, I can't put anything in the back face image well. I have tried TIFF, JPEG and even DV quicktime video. They wont stick.
Are you dragging the backface image straight from the File Browser to the image-well? If so, that's your problem. Any optional inputs (image-wells) for filters, image masks, particles, etc, require footage that has already been added to the project.
You can either drop the footage into the Layers List and disable it (to hide it), or you can drop the footage into the Media tab. The Media tab is like the clips bin in FCP: it associates footage with your project, but the footage isn't necessarily IN your project. You can then drag stuff from the Media tab into the Canvas, Layers List, Timeline, image-wells, etc.
Also, any files that are in the Media tab are cached and taking up RAM. If you imported a file but later decided not to use it, remove it from the Media tab to improve performance. Don't remove anything that is being used in any layer though!