I bought one years ago and never really quite figured out how to make it work. I am much more experienced now and discovered that I can do the "curve master moves" without the curve master. The technique works just as well with the feet that came with my machine.
It does help to have a knee lift. I put the concave piece on top, and hold it up in the air so it's not touching the bottom (or convex) piece until it goes under the presser foot. As I sew, I pull with a tiny bit of resistance on the top piece, just barely stretching it. Occasionally I need to stop, lift the foot, adjust the patches, then continue sewing. This takes some practice, but I've gotten good enough that I know just how much to pull on the top piece. 99 times out of 100, I sew from start to finish without pinning and my edges match.