Some configuration to your account must be done at Authorize.Net's Web site (Authorize.Net will have supplied you with access details). This is referred to as Authorize.Net's Merchant Management System. Login at:
Go to 'Settings and Profile', under 'Security', select 'obtain transaction key'. Enter your secret answer to obtain your key. This key must be entered in the TransactionKey setting (see @21.3.1. Setup within Kartris).