Usually those things have a website you can go to to register a billing address so you can use it on line. Try reading the packaging that came with the card if it isn't printed on the card itself.
i did go through the packaging/website etc. No registration required. Here's their website http://usa.visa.com/personal/cards/prepaid/gift_card_how.htm...
> To make an online or phone purchase, you will need to register your card. You can do this by either calling the issuing provider, or by visiting their website. This information should have been supplied to you when you received the card. It may also be printed on the back of the card. When you make an online or phone purchase, the name, address, and phone number you use will need to be exactly the same as the information you provided when registering your card.
http://usa.visa.com/personal/cards/prepaid/gift_card_how.htm...