Its actually REALLY easy, its something like...
Load itunes as normal, log out, click my store thing at bottom of shop page to switch to us store, then go through the usual registration of an account as you do with itunes normally, when it asks for an address, be creative, I used the british embassy. Itll ask for funds to create the account, the far right option is 'redeem', chuck your voucher code in there and voila, US account with US dollar voucher