Access the casino and enter the bank
Choose Bancomat as a payment options
When asked for invoice details simply click ok (with Postepay you are not going to receive any invoice, while this may happen when using other credit cards)
Click "register your card"
Now you can deposit money in your casino account simply inserting your CVC number (the 3 number digit you can find in the back of your card) and choosing the amount you want to deposit.