There's a convertable free motion foot you can buy, it has different attachments for the openings like a closed hole piece, open hole (like the one that comes with your machine) and a large plastic bullseye hole piece. They just screw in to the main foot.
It is not necessary to have that to use the special bobbin case. I had the 6600 for awhile and I didn't get the special attachment until I got the Horizon.
I found that you definitely need to adjust the tension on top though, the new bobbin works pretty well.