When you arrive you are directed to one of the campinggrounds and there you simply choose a spot to put up your tent. It's really as easy as that.
Usually things at Wacken never got stolen, but on some other festivals in Germany the phenomenon of theft has appeared for the first time this year. Even things like cutting up the "walls" of your tent and then stealing whatever might seem worth it. Partially even whilst the tent owner was sleeping in there at the same time - so they must have used some kind of narcotics... I don't want to frighten you, but it seems this is a new, growing problem on festivals. So: Keep your money and really important things (ticket on the first day, PASSPORT) close to your body or use one of the lockers that you can rent for a small fee. Keep your eyes open and try not to pass out totally drunk somewhere because this is the easiest way for someone to rob you.
You can already have a look at the maps for this year's festival:
http://www.wacken.com/de/woa2007/main-info/plaene/
On this picture
http://www.wacken.com/typo3temp/pics/0e86af5d97.jpg you can see the whole area including all of the campgrounds.
http://www.wacken.com/typo3temp/pics/0eab510aaf.jpg Here, number 27 is the place where you can rent one of the lockers.
These maps will probably also be in the festival program which you can buy for...well...probably one euro again at the merchandising stalls. But you can have a look at the pix and already try to get used to where you have to go so that you don't get lost.