After Logged in when you click on a course name you will be sent to the course enrolment page. Here you will see 2 options:
First is Payment Gateway with which you can pay the course fees and enrolled in the course.
Second is Enrolment Key If you have already made the payment then you had received the Enrolment Key in your Email. You have to enter that key in the "Enter Enrolment Key" box and click on "Enrol me" button.