To protect the authentication key that you must use as part of each transaction request, you should not store this key within the actual App code. Other details, such as your Store ID, can be held within the App code.
We would advise that your App obtains the authentication key by downloading it from your servers as part of either an installation or registration phase. This process must be over a secure encrypted link (for example, HTTPS). Once the App has the authentication key, it should store it in the device in a secure method, such as using the iOS Keychain.
This method also allows you to update the authentication key should this be needed.