To purchase city cash, you'll want to look near the top of the app screen, where it tells you how many city coins and cash you have. Under the cash counter, you'll see a link that says, "Add Playfish Cash." Click this link and it will open a window allowing you to choose the amount of cash you want to purchase with various methods of payment, like a visa card for instance. After you complete the transaction, the city cash will be immediately added to your account. Hope that helps!