I've been having some Authorize.net issues myself, but as far as setting it up all you need to do is go to courses / enrolment and then you'll see a screen that gives you several options, the internal enrolment should be selected by default. to change that click authorize.net as default and enable it, you'll also need to go into settings which is right next to the default option. after you've clicked edit, you'll see a screen asking you to enable the HTTPS. make sure that HTTPS or SSL is active and enabled on your
server, if its not, then YOU WILL BE LOCKED OUT OF YOUR SITE! This is very important that you have HTTPS enabled on the server first before enabling it on moodle.
once that is taken care of you'll be able to access the settings for authorize.net you'll need to add your username and password and make any changes to the settings as you see fit. I hope that helps
