Step 1
Go to this website
http://www.wacken.com/en/woa2008/
Step 2
Choose "Tickets/Shop" from the top menu, and in the fall down "Tickets"
or go straight to the followingwebsite
http://www.wacken.com/en/woa2008/main-tickets-shop/tickets/
Most of the information is still in regards to 2007, I guess they haven't released more yet, that would probably mean the people on this forum doesn't know much more either.
If you follow the link from the Wacken page to the Metaltix page you can find the 3-day WOA ticket among the best selling, or go straight to it using this link:
http://www.metaltix.com/item_detail.php?item_nr=23399
The information might be in German, but write the number of tickets, press the button <put into cart> and then go to the "Shopping Cart".
Press "Order Prepare" and then check in or register for a new account.
You may also use the "Contact" link at the bottom of the page if you wish them to contact you.
Basically not much different in comparison to other online stores.
Do count on a rather high shipping cost though, and make sure you have a Mastercard or VISA for payment.
Prices in German you can find here:
http://www.metaltix.com/index_faq.php
I'd guess a ticket to the US would be 10 Euro.