As far as I know cmpass.com seems to be the most reliable website for topping-up. I've used it once and it was very quick and fairly easy. The simplified steps I took were:
Sign up with cmpass.com
Use paypal to pay txtNation for $10 (can be done multiple times)
Get pin code and claim code
Insert into allTunes
For a more detailed and specific set of steps feel free to follow vikulenka's post above
