Yuu could also try
http://www.arena.net/about/contact.html
If you don't mind our asking (as I'm certain many people are curious), what is the problem you have? I ask because typically, the only reason an individual to contact a gaming company is due to either copyright/patent/trademark infringement issues. Or they feel that the game, the ads, etc don't live up to their expectations about what they were supposed to get and they plan to sue. Of course, there is also the Jim Thompson excuse, the game is evil and must be purged, but I don't think Jim Thompson cares much about Guildwars, his favorite target is GTA
In either case (except the Jim Thompson reason), the user base is usually a good starting point to determine if you actually have a case or are just under a misunderstanding of some sort, and when it comes to legal issues, misunderstandings can cost you $$$