Enter your merchant account information so that you can accept credit card payments from your customers via your portal.
In Method CRM, go to Customize > My Account.
Click view / edit... beside Payment Gateways.
Click Add New Payment Gateway....
Follow the steps on the screen and then click Add. NOTE: if you use Intuit Merchant Services, there are special instructions you can find here: http://miurl.cc/PGateway.
Test out your payment gateway by paying an invoice under Customers > Receive Payment.
Authorize.Net requires an API Login ID as well as a Transaction Key to be submitted with each transaction going through their gateway. These are required fields during the Payment Gateway setup process in Method. Read the Authorize.Net Access Settings article for instructions on how to obtain your API Login ID and Transaction Key.