It normally takes only a couple minutes for the credit to show in your game, but it can take up to a couple hours. Try refreshing your game to update your total if it hasn't shown up after five minutes or so. If that still does not update your credit, click the Earn Mall Credits heart icon again and find the small customer support link in the upper right corner. This will take you to a page where you can get in touch with Trialpay's customer support. Let them know you did the offer and have not received your credits!