a) make subtitles as b/w pic with res of your movie size.... open bth in QTpro; select all, copy; switch to movie; add scaled; switch to movie properties; choose track 2; choose mask... now, your titles shoudl be superimpsoed....
BUT
b) open mov in iMovie... select titles.... iMovie is pre-installed on every Mac, why the hack using unconvenient QT?