Hi David,
Have a look here.
The Idea is to have the motion based on the camera as well the objects expressed in (camera view) x and y. No move equals like 50% gray. It is stored in two channels of the RGB (Blue is not in use).
In the RenderSettings.Options you find a Motion Scale parameter. As each scene might contain a different scale and speed, this parameter is critical to set up, so the color channels work well and express the movement spread out over the whole scale of available gray values in each channel. If that is not set up accordingly to the scene, you might have only black and white in the file or just some derivation of the mid-tone gray.
In the Plug you will find another option to “scale”, but be aware that none of the both is able to fix problems from the other, both need to be set up carefully.
Again - as this is a data channel, it needs to be with out Antialiasing and it should not be used with 8bit/c. make certain that the material comes into Ae as RGB values and not distorted with a color profile or a gamma adjustment.
This method has the typical shortcomings of a post based effect and will not reach to the quality of the physical based render options in R13. However, I recommit highly to not use integer based footage, as motion blur lives from in most cases from values above 1.0, which is most likely not available with 8 or 16bit/c integer.
All the best
Sassi
