Microsoft and windows phone has extended its "get $25 prepaid app card" free program to Feb. 14th.
All you have to do is submit your cell phone number and IMEI number for your Lumia phone HERE, then follow the steps, fill out your mailing address and send proof of purchase and copy of receipt.
it should take about a week for you to get the $25 card in the mail.
Use it to spend on apps, games, xbox live games from the marketplace.
i already submitted mine about a week ago and got it:
|My $25 prepaid app card|
once you get the card, use it by going to the marketplace,
pick the app/game of your choice,
choose "change payment method"
enter the card details
and you are good to go!