If you add an employee to a Time Off Accrual rule, the rule will not automatically apply to previous pay periods.
To add accruals to a previous pay period you would need to visit Employees in the top navigation and select View next to the employees name. Once in their profile, select Time Off on the left hand side. This will take you to the Time Off section.
Once at the Time Off section, select +Add Time Off Accrual Entry. This is a one time entry that will allow you to add or subtract Time Off from the selected Time Off type.