have you ever taken the shank off your machine? if it's not a really old machine, to change feet you just press a little button and the foot falls off. to remove the shank there is a screw on the left (not the one that holds the needle in place) loosen that screw (or just remove it) the shank comes off, now put the walking foot there so that the fork wraps around that shaft where the other one was, the walking foot has an arm that must be on top of the needle screw, now thighten the screw on the fork. it should work fine.
good luck