Take the top off the agitator, or fabric softner dispenser assembly by pulling it straight up. Then down below you'll probably see a semi-translucent cover with a black ring around it. Pull this straight up. That's a water seal to make sure the bolt that holds the screw in stays dry.
You should now see the screw that holds the agitator down in the middle of the agitator. If you don't and only see a hole where one goes, order the one that
Yogi linked to. Put the screw in, the semi-translucent cover (it snaps in) and the fabric softener cup and you are done.