okay listen well.
when you enter the world that never was
you will eventualy get to a place called memory skyscraper
there you will see a cut scene when you FIRST visit
a cut scene showing roxas (in a black robe) and sora fighting
when this scene is over you will have the chance to get final form
all you have to do is constanly use drive. valor wisdom etc... final form will appear like antiform...........randomly....without warning
but once you've accidently changed into final form
you have unlocked it and you may use it whenever you want.
(you may want to max out your drive form levles while your at it)
if you have any questions about what i've written....pm me.