Before you can assign officials to events, you'll need to make sure you've setup roles for them first.
Set Roles for Officials
Go to Officials > Roles
Click Add New Role
Enter the name for the official such as Linesman or Timekeeper for example and how much they'd be paid for officiating an event
After adding all of your roles, go to Officials > Divisional
Toggle through each of your divisions that use officials and enter how many of each role are needed per event.
You will now be able to assign officials to events and manage their payout amounts.