EUREKA!!!!!!!!!
http://www.sewingmachinesplus.com/P60498.php
just tried it for the first time. i just had to adjust my needle position to find the center of the foot (which is because of the machine, not the foot. i always have to do that). it's super-easy to see whether or not your edge is lined up with the foot.
i even tested it for ditch-stitching on a sample sandwich. dead-center. if i didn't know where the stitching was, i wouldn't know where the stitching was! it isn't a "proper" quilting foot; i had to pull gently from the back to keep it moving smoothly; but once i got the hang of it i was amazed.