The short answer is: you don't. When the year starts, all of your students are already enrolled. After that, adds are processed automatically every night, so you really shouldn't need to enroll anyone manually.

In the unlikely event that you do need to, however, it's pretty easy. From the Control Panel, click on "Enroll User" in the "User Management" pane. Search for the student by last name, check the box next to their name, and click "Submit." Sometimes, the user doesn't appear when you search for them; here's why.