The chips 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 chips, but we just need a copy of that email.
Once you have that receipt, go back to Blackjack and use the Email Customer Support option located in Settings, attach the receipt to the email and send it to us. We can look up your account and get the chips credited to you.