Go to the bindings panel and click the plus button.
If you are using the Authorize.net AIM Method, select eCart -> Checkout -> Local Checkout response. From the gateway list select Autorize.net AIM
If you are using the SIM Method, select eCart -> Checkout -> Checkout Form Response. From the gateway list select Autorize.net Post.
This will add a binding group to the bindings panel that you can use to show the error message, or error number and other details.