if you have opened it, take it back to the store and tell them the dvd was scratched or not playing properly and that you would like to EXCHANGE it for the MORE EXPENSIVE two disc dvd with the special features. I suspect you shouldn't have a problem doing that. As Matt G pointed out, if you haven't opened it, the store will certainly let you exchange it in upgrade for the two disc dvd.
"Blow out the candles Robert and make a wish. Want something, want SOMETHING."