Make sure you are threading the needle from right to left (ask me how I know that one) and make sure the presser foot is down- easy to forget that one too. Drop the feed dogs, put the stitch length lever on the middle line and then try. If that does not work, take the darning foot off, raise the feed dogs and put on a regular foot and try to sew a regular seam- maybe the bobbin is backwards (gee, I sure made a lot of mistakes when I set up my 301 to FMQ!)
Good luck- I love FMQ on my 301. I have that plastic darning foot and it works fine on mine. Just thought of one more thing... when I put that foot on my machine, I thought it didn't line up right- it actually -did- once I had put the presser foot down.