Click the green 'Search' button,
Search for the course you are trying to add (make sure to uncheck 'Show Open Classes Only'),
Click 'Select Class',
Check the box: 'Wait list if class is full',
Click 'Proceed to Step 2 of 3',
Click 'Finish Enrolling'.
The only way to enroll in a full class is by placing yourself on the waitlist.
Students are moved from the waitlist into the course only if a seat becomes available
(in the order of your waitlist position).
Placing yourself on a waitlist does not guarantee enrollment.
Factors that may prohibit you from being placed into a waitlisted course include
unit limits, class time conflicts, prerequisite issues, etc.
If you are on a waitlist once classes have started and would still like to try to
you'll need to “crash” the course (see "How to 'Crash' a Class").