There are several ways in which you can enroll a Student to a course.
You can go on the left hand side menu and head over to the Users tab, then edit the user you wish to enroll.
Scroll down and click on the course(s) you wish to enroll him into.
The other method is to navigate in the upper menu to the Courses tab and from there Edit the course you want your student(s) enrolled into.
Scroll down to the Enrolled Students section, in the field box type their name and then click on the Add New Student button.