Within the Mobile Platform - 

The Registration Settings for the master account is were the Auth.net API Log In and API Transaction key are entered and stored. 

Step 1.

Click Client Admin from the main menu.

Step 2.

Locate and click the Registration Settings button in the upper right-hand side of the page.

Within Authorize.net - 

The required API keys are found within the API Credentials and Keys menu of the Authorize.net account. 

API Login Key

API Transaction Key

For security reasons the Transaction Key is hidden from view. If the original was not recorded outside of Authorize.net, a new key will need to be requested. When a new Transaction Key is requested, the previous key will be disabled immediately, or within 24 hours. Any external system still linked to the old transaction key will fail to process payments once it is disabled.