If a permanent employee is on leave, and not working on a public holiday you can enter this under Pays >> Edit Pay >> Edit Leave Taken using public holiday as the leave type.
If an employee is working a public holiday and requires loading on their standard pay rate you can set up a new, secondary pay rate under Employees >> Pay Settings >> Special User Defined Pay Rates.
To set a public holiday rate up for multiple full time employees, go to Company >> Pay Rate Groups >> +New Group and give the group of rates a name (eg. Public Holiday Rates). Next, click on the green plus symbol on the right and give the rate a description such as Public Holiday, leave all other settings as they are, and click Save. Finally, add all employees who will need this rate using the green plus symbol below the rate you have just created. Now, when you go to pay a public holiday for a full time employee, you will select a second line of pay in Edit Pays and select your new pay rate, Public Holiday, and enter the amount of public holiday hours your employee did in this particular pay run.