1. In the left-hand side menu, click "Academic". Choose a class and click "Actions" > "Manage Attendance".
2. For every lesson you have, click on "Set as taken" button. This means that teacher confirms all students are present in the class. If the student is absent, set it's status accordingly by clicking "Excused", "Unexcused", etc. As soon as you set student's status, the lesson is automatically set as taken.
Watch this example on how to set attendance for a student: