You may buy Facebook Credits from the Payments tab in your Account Settings or directly from within any game. You can purchase Facebook Credits in your preferred currency using a major credit card, PayPal, your mobile phone and many other payment methods powered by PlaySpan.
If you try to buy something in a game or app that costs more Facebook Credits than you currently have in your account, you will be prompted to purchase more. The item's cost will be deducted from the number of credits you purchase, and the remaining amount will be added to your balance.
To see a list of available currencies, go to the Payments tab and click "change" next to "Preferred Currency". Purchases in non-US currencies are first converted to US dollars using the current exchange rate, then Facebook Credits are added to your account at USD $0.10 each.
Depending on your mobile carrier, the mobile payment option may only be available for purchases under $10.00 of credits.