You go to the Wizards of the Coast website, click on the bottom left 'Help' link, then the tab that says 'Email Us'.
There are also several frequently asked questions for various products there.
I will tell you though that for purposes of asking rules questions, typically a customer service representative will give you his own interpretation of the rules. Quite often if you ask the same question multiple times, you will get different customer service reps answering it, with different answers. Just take what they say as the advice of another player unless they specifically say that the rule will be coming out in an upcoming FAQ or something (they often say they will forward it for an official answer but that's not the same thing).