On your summary page, click the class name in the class table. On the class details page, you will see a table of assignments, quizzes, etc. The class details page is split into two tables, past schedule items and current/future schedule items. In the Options column, click the "Assign Grades" link. Enter a grade and comment (optional) for each student, then click the "Submit Changes" button.
See Also: Grade Computation, Report Cards, Schedule Assignments