Details for enabling transaction api can be found at:
http://developer.authorize.net/api/reference/features/transaction_reporting.html
We recommend testing the configuration before continuing as sometimes the existing API key will not work. You can test that the API key works correctly by triggering the ARB Recovery feature in Atlas. In Settings (click Admin -> Set Preferences -> Application Settings), on the Credit Card Processing tab, click the “Recover ARB” button. If it is successful, it will inform you of transactions that have been processed or that no transactions are missing. If there is an authentication error, create a new API key on Authorize.Net and configure the gateway in Authorize.Net to use that key.
Note: The generated key on Authorize.Net might not always be compatible with the Transaction API. Changing the API key again should resolve this.