It's not a darning foot. It is a stocking darner. You put the stocking, or other item to be darned, into it and tighten down the "coils". Then it goes under the foot - the actually darning foot. The plate is elevated with the control on the bed of the machine. Set it to fine, and away you go. Make sure it is set right in the middle of the "fine" area.
I see you also have a button foot and a zipper foot.
The cabinet and machine are both worth restoring. You could make them shine again. They deserve it! If you have trouble with the machine, it will need to be serviced. I'm not sure if I can tell you how to do that over the internet, but it will be worth the money to have it done locally.
Great machine! One of the very best.
Cricket