You first need to enable referees in the system.
Do this by going to Setup > Settings > then use the 'Roles' tab.
Then tick the checkbox for the referee / official role.
You then need to create some people and add the referee / official role to them.
Do this in People > List People > Referee
then press the 'Create New' button
Add their details, and select their role to be a referee / official, then press create.
You should then be able to add this referee to a match by going to:
Referees > Assign Officials
Here you will see a list of matches where you can assign referees / officials to the match.