You can set up the ABN or VAT or the TIN number of your organization using the "Additional Fields" under the organization profile settings.
For example, if you want to update the VAT details (same for ABN or TIN numbers) of your organization, you can follow the steps given below:

• Click the Settings tab.
• Under Organization Settings, click the Organization Profile link.
• Scroll down to Additional Fields, enter "ABN Number" in the Field Name and update the actual ABN number in the Value column.
• Once this is done, click the Save button.

Here's the screen-shot with a sample scenario:

The details entered in the additional fields will reflect under the organization details section in the invoice PDF.