According to the manual (PDF), your Brother machine is much like mine in this respect. On mine, the feed dogs can't be lowered. However, among the various doodads and fiddly bits that came with the machine there's a small plastic plate, the darning plate. In order to free motion quilt, you should slide the darning plate over the feed dogs. Page 44 of the manual has a picture of the darning plate and how to put it on.