Foreign gift voucher not working

I was given an iTunes gift voucher from the U.S. I'm australian and after poking around I realised I had to make a U.S. Apple ID. Once I had done this when I tried to purchase an item with this card, it says "Your purchase could not be completed" and behind that it says 'unable to connect to the Itunes store'. I'm not sure if this is a fault with me or with apple. Can anybody help?