You can use it anywhere on the internet that accepts Mastercard (pretty much EVERYWHERE) or for a small fee (less than $10) you can get a physical Mastercard that you can take to any (physical) store (or any other kind of business) that you'd like.
Thanks for the help Tina! But wouldn't you know it ... it says its only for residents of the US .. d'oh! Would have been nice to use it for the hotel reservations...