When you have casual employees who have not worked in the period you are processing, ignore these employees, and they will show up in the drop down in Wages. You cannot save a timesheet with no entries.
If you have an employee who is not getting paid due to Annual Leave or Sick Leave running out or an employee who has taken Leave Without Pay, in Wages, on the Adjustments screen enter the whole period's date (including Saturdays and Sundays and any days that he does not normally work) as Leave Without Pay. This will create a record of the dates he was on unpaid leave, and it will also reverse any leave accruals that may have occurred for this period.