The gems you purchased should have automatically been added to your balance.
If your balance was not updated after your purchase, it may be because of a problem with the Apple Store completing the transaction. Apple should have sent you an email confirming the purchase. We can credit your account the gems, but we just need a copy of that email.
Once you have that receipt, go back to Blackjack and use the customer support e-mail, located in Settings, to attach the receipt and send it to us. We can look up your account and get the chips credited to you. To look up your account, we need the information automatically included in the "email customer support" which includes your Account ID number.