Its a buttonhole attachment. The attachment opens from the underside and the cams (metal things) fit in. The cams determing the shape of the button hole. If they don't immediately fit properly, turn the knob to move the "thingy" inside so that it sits at the proper location on the cam, close the bottom. From behind the machine, attach the forked looking thing over the screw that holds the needle in place. In other words, the green part should be in the back with the metal part closer to you.
Now put a double thickness of fabric under the metal part of the attachment. If the machine is zigzag, set your zigzag width to a medium width and set you stitch length to medium also. You may not need to do this, the attachment may regulate this for you, I don't remember.
Anyway, this is the basics of this older approach to buttonholes. I hope this helps a little.