When you are in a class, swipe from Left to Right across your screen to bring up the Sidebar. Here you can mark Attendance, record Achievement's & Behavior, as well as access your Marksheets.
To take the register, tap on Attendance. Then, tap on all of your students who are present to select them, and then tap on Present on the sidebar. These students will now be marked as present! You can repeat this step for any pupil who is Absent or Late also.
If a student is Absent for a particular reason, you can tap on Code and then specify the reason for that student's absence.
Don't forget to press Save to record your register in SIMS!