The coins 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 coins, but we just need a copy of that email.
Once you have that receipt, go back to the game 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 coins credited to you.