So now that you've gotten your airfare money set up and ordered your ticket from Metaltix, you need to set up your trip logistically. The important decision is when you want to go over, and when you want to come back. If you can't get much time off work, you'll want to fly out Wednesday after work; given the flight time and that Germany is six hours ahead of the East Coast, you'll get in to Hamburg in the morning on Thursday and over to the festival in time to get set up before the bands go on. Similarly, you'll probably be able to get a flight back from Hamburg by Sunday evening, which will allow you to at least technically get back to work on Monday.
The problem with this is that if you don't sleep on the plane, you will be dead from jetlag and almost definitely miss some bands on Thursday night. If you don't care about Thursday's bands, fine; if not, and if you have enough vacation time, you may want to try flying over after work Tuesday. You'll have a better chance at a good tenting spot, and also get some time to acclimate to the timezone and/or see TSV Wacken get their butts kicked by the St Pauli Old-Time All-Stars. Of course, the appeal of this game is limited for people who aren't familiar with German soccer, but there is beer and cool music, and it's a decent way to get an advance look at the culture of the festival. Similarly, if you can take Monday off and have a little room in your budget, you may want to put up for a hotel room in Hamburg and catch up on your sleep before you fly back -- and also to wash all the grime and grit of the festival off. There are washing facilities onsite so that you don't make your transit vehicles totally unlivable, but they're nothing like the real thing.
Ok, so let's say that you've got all your travel arrangements lined up. You find yourself in Hamburg, but Wacken is actually in a whole different federal state, more than an hour to the north by rail and road. This is how you get there.
We'll assume you start from the airport in Hamburg. After you clear customs and immigration (notes on the passport process will follow later), find an ATM to get out your first allotment of cash. Look for a sign like this:
Now that you have your weirdly-colored Euros, follow the directions to the bus to the main train station. Directions are posted in English as well as German, but the bus sign may not be; the huge German word to look for is 'Hauptbahnhof'. This may be abbreviated 'Hbf' elsewhere, but this bus is a special that just goes to the train station, so it'll be spelled out. This bus costs 5 euros and is the simplest way to get over.
When the bus drops you at the train station, go inside and start looking around the tracks for groups of metalhead-looking people. These long-haired, black-shirted crazies are your natural compadres, even if their native language is German, and more importantly, they know which trains are going the way you need them to. If you find yourself on your own, you'll need to look for a train going through Itzehoe, or, failing that, any train stopping in Altona. Itzehoe is the station stop for Wacken, and Altona is another train station in Hamburg that handles most of the regional connections from the north. At Altona, look for a train going through Itzehoe. It should be noted that going through Altona, you'll be on regional rail rather than an inter-city express, which will naturally be cheaper. Depending on train and travel class, the ride from Hamburg to Itzehoe will cost between 10 and 25 euros; get in a car with a 2 on it if there are numbers on them. These are second-class accomodations and cheaper. You may have to change trains at a place called Elmsdorf; in this case, just wait for the Itzehoe train. This town is pronounced almost exactly like "it's a ho", so if you're listening, it's tough to miss. The inter-city avoids the train change.
Ok, let's assume that you've overcome the German rail system and gotten off at Itzehoe. When you walk out the front, there should be a sign along the road to your right in the shape of the famous steer's head, and probably a clot of metalheads standing around waiting for the shuttle bus. This bus will take you right to the festival gate, and costs 3 euros. Now all you have to do is join the lineup and get your wristband and the swag bag that goes with your camping fee. For the latest on how this is going to be managed, see the official Wacken site; there was kind of a clusterfuck with the bag/fee situation in 2006, so there may be changes going into the festival.
Going home, you pretty much just reverse field: on Sunday morning, you take the Wacken-bus to Itzehoe, then take the train to Hamburg, usually changing at Elmsdorf and then again at Altona. From the main station, you can take the airport shuttle back to HAM, or make connections to wherever you need to go. The bus will probably say 'Airport Express'.