To use the TurboTax Federal Free Edition you must have a US address to register the program.  Since the version of the program is free, registering takes the place of paying for the program.

If you upgrade the program to one of the paid versions it will allow you to file with a foreign address, but you will need a US based card with a US based billing address to pay for the program.

This is due to IRS regulations imposed on TurboTax to assist in maintaining security of accounts.