In order to configure Authorize.net CIM for your ClientExec installation, navigate to Settings -> Plugins -> Payment Processors. Choose Authorize.net CIM.
You will get something like this:
Your Authorize.Net CIM API Login ID and Authorize.Net CIM 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.
Demo Mode will allow you to sandbox your ClientExec installation. In other words, you can order products, choose Authorize.net CIM as the payment method, and no charges should occur. Remember to uncheck this option when you are no longer testing.