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 MC Trade. In Settings (click Admin ->Global Settings, then Integrations, Credit Card Gateway), scroll down to the ARB Recovery section. Enter a date range (recommend just one or two recent days), and click Import ARB Transactions. If it is successful, it will inform you that ARB Recovery has started with a green notification. 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.