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.
The pack with the best price-performance ratio is displayed in the top left corner. In the top right corner you will see the pack that has been purchased most frequently. The remaining packs are listed below.
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.