Click the Buy Symbol at the top right next to your account balance to get to the GameTwist shop. All available Twist packs are listed here.
To purchase a Twist pack click the Price button next to it. You can then confirm your purchase in the dialog window that will be displayed. The twists 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.