Click the Plus Symbol at the top left next to your account balance to get to the Slotpark shop. All available Credit packs are listed here.
To purchase a Credit Pack click the Price button next to it. You can then confirm your purchase in the dialog window that will be displayed. The credits you have just purchased will then be added to your account. A few minutes later you will receive a payment confirmation from the Google Play Store, respectively at the end of the month from the iTunes Store.