Hmm I'v been to a few so I'll explain how they work at the ones i go to.
You all sit down at tables and every person is given 6 packs. So yeah you open them up and make them into a deck, of which you are given as many basic energies as you require but not basic dark or metal
Once you've finished deck building an organiser will check your deck to make sure it's 40 cards etc. HOWEVER you arn't alloud to change your deck at any time during the tournament, and you arn't aloud to trade the cards you've been given until after the tourney is over
Next you get divided up into pairs and play each other at 4 prize matches. This happens 4 times and at the end, the organisers will announce everyones rankings. NOTE: they are usually divided into 3 categories, Junior, Senior and Masters, according to age groups. The top 3 in each age group recieve extra prizes, to add to the 2-4 more packs that you'll be given at the end anyway. Also, you can sometimes recieve a deck box/sleves/PRE-RELEASE stamped promo card/notebook or a badge.
Then after its all finished, well its all over, and you can freely trade the cards you got with other people and generally hang around and stuff.
EDIT: There is usually a time limit on matches, and while the others finish their matches, can can do what you want, like play against others with your real deck and stuff.
Well, thats all hope it helped! I'm also attending a MD pre-release on the 17th of May so I'll post and tell everyone what it was like