Find "Buy Coins" button at the lobby of the game page. https://apps.facebook.com/doubleluck
Select the amount you would like to purchase and click "Buy Now Button"
Facebook games are requred to offer payment through Facebook Credits only. You may use credit cards, PayPal, mobile, gift cards, and other similar payment methods.
To learn more about Facebook credits, go to: https://www.facebook.com/help/147418788728482/