Hi. Where you are is at the set up for tax codes and not where you control taxes per employee. You need to go the employee card, payroll, taxes and add any extra tax an employee wants deducted there or change to the correct tax status if not currently set that way.
If I am reading it correctly, if you or employees need to pay PAYG instalments - they are a separate extra tax requirements to be paid by the individual employee on top of their wages tax - it is the ATO requesting prepayment of taxes in advance if there was a shortfall. This is a separate obligation to taxing an employee's wage in MYOB.