Go to the event page. You can do it either from the dashboard (it will display the latest published event): or by going to the statutory listing page (1) and clicking either "Go to the event page" button (2) or the event name (3).
Press "Manage incoming info". If you don't have it, that means you don't have enough permissions in the system:
You'll see a table with users who are accepted and not cancelled. If someone's not there, either this person didn't apply, he/she is not accepted or the application is cancelled.
To mark participant as confirmed/not confirmed (e.g. when he/she has paid fee), press the select in the row for this application in the 'Confirmed?' column and select 'Yes' or 'No':
To mark participant as attended/not attended (e.g. on the registration desk), press the select in the row for this application in the 'Attended?' column and select 'Yes' or 'No':
Keep in mind that you won't be able to mark participant as attended without marking him/her as confirmed, you have to mark this application as confirmed first.
Also keep in mind that you won't be able to mark participant as not confirmed if he/she is marked as attended, you have to mark this application as not attended first.