The way you would open a door with a credit card is to disengage the holding mechanism that is part of the door knob. You would slide the card into the crack by the door knob and bend it away from the door knob while pushing on the door.
From what I've seen on movies and in real life all you need is a credit card and a door that does not have a deadbolt. Then you insert the card, jiggle it around a bit and somehow the door magically opens. I wish you good luck in your attempts.