From GroupScript documentation
To enable Authorize.net support for our software, you need to sign up for Authorize.net merchant account, and have AIM (advanced integration method) available. To modify config file, you will need:
1. Authorize.net transaction key 2. API login id
These details must be entered in Backend->Settings->Payment options->Payment authorize tab.
If you are using old version and don't have the Payment options menu item, the configuration has to be entered in system/application/config/authorize_net.php