I have not seen any walking feet that snap on. I've seen several different ones and they screw in where the ankle piece goes - this is the part that the snap on attaches to. I don't know about your particular machine, this is a general comment on what I've seen.
So what happens is you unscrew the ankle, screw on the walking foot, making sure that the right side lever (not sure of the name) goes over the screw that tightens the needle.