The knob (aka presser foot holder) that needs to be removed is on the machine, not the attachment. It is the knob where you put the presser foot.
There is a knob to turn to tighten up the foot, but there is a screw that is on the side that if you loosen that, the turn knob can be removed and you will have a round presser foot bar visible. Then you can put the attachment on the bar. Yes, this is for the buttonholer, but it applies to the overcaster as well. You do not need to cover the feed dogs for the overcaster.
Janey - Neat people never make the exciting discoveries I do.