If you are registered for GST, then when you download the GST Worksheet under Localisations then the ABN custom field should appear but I note that that is not occurring.
If you aren’t registered For GST then create a custom field as others have advised.
@lubos, if a user is downloading the Australia GST Worksheet then that automatically implies that the business must have an ABN (as you can’t register for GST without one), yet under Setting > Business Details no ABN custom field is created with the GST Worksheet is download. However, if the user downloads the STP, Worksheet, then the ABN custom field is automatically created under Setting > Business Details with that download.
As a business is more likely to have GST obligations (due to the turnover thresholds) before it has Payroll (paying employees) obligations then the ABN custom field’s creation should also occur when GST Worksheet localisation is download.