From your 'My Hub' page (please see below)
On the left hand side of your account, you will see the side menu where you can select 'Calendar'.
This will show you an overview of that month's sessions/events
Here you can click on the event followed by the 3 horizontal grey dots to 'View'
If you hit the 'Start Event' button at the bottom of your screen
You will be able to take a register and check all attendees in.