#1 if you are purchasing courses you need to register to set up your student/user account. Your account gives you access to the courses you have purchased, and other members only resources. you can register here or at the check out when you are ready to pay for your first courses.
#2 Log in means to Sign in. You must be signed in to access your courses, to purchases new ones and to review your records or update/edit your account information. You have unlimited access to the courses you have purchased.
#3 To add new courses to your account; Sign in (Log in) then go to the Courses Drop down Menu. and select the courses you wish to purchase. Alternatively you can go to the student store and purchase the courses from there.