Plastic square with an arm sounds like a walking foot. Your feed dogs need to be up for it to work properly and the arm needs to be over the screw to tighten your needle. Darning feet or Free Motion Quilting feet are round.
Walking foot
[ATTACH=CONFIG]54447[/ATTACH]
Plastic darning foot
[ATTACH=CONFIG]54509[/ATTACH]
Metal darning foot
[ATTACH=CONFIG]54510[/ATTACH]