To take individual attendance, you must first open the data event. When the event is open, scheduled students default to present. To mark a student absent, select that student on the left hand side under Students. You will then click 'Actions' and 'Mark Absent.' Next, indicate the reason for absence using the drop down options or choose 'other' for a customized reason. This information will then appear on the student's attendance report.
You can also mark all students in a group absent for things like a fire drill or holiday - that is done in the agenda view on the daily data screen using the 'Actions' button next to the group.