In order to configure Authorize.net for your ClientExec installation, navigate to Settings -> Plugins -> Payment Processors. Choose Authorize.net.
Your Authorize.Net API Login ID and Authorize.Net Transaction Key can be obtained by logging in to your Authorize.net account. Click the Account tab. Scroll down, and find "API Login ID and Transaction Key" under Security Settings. You can also create your MD5 Hash here by clicking "MD5-Hash".
Demo Mode will allow you to sandbox your ClientExec installation. In other words, you can order products, choose Authorize.net as the payment method, and no charges should occur. Remember to uncheck this option when you are no longer testing.
You can individually allow the four credit card types (Visa, MasterCard, American Express, and Discover Card) to be accepted.