vBulletin Manual - Authorize.Net

1	Log in to Authorize.Net merchant account
2	Click Settings on the left menu
3	Under the 'Transaction Response' group click Relay Response

You should now be presented with a screen containing the following:

You should now enter the following URL

https://www.example.com/forums/payment_gateway.php?method=authorizenet

Click [Submit]

You should now be on the main Settings screen again, click 'Obtain Transaction Key' under the Security group. Enter the appropriate secret information and a new key will be generated for you.

Note:

If you already know your transaction key you dont need to generate a new one

To set the Login ID and transaction key go to Paid Subscriptions > Payment API Manager > Authorize.Net and fill in the fields with the appropriate values.