Argos 8 introduces a new change in that tax rates are set on your Printer Locations rather than your Media and Costing settings.
If you use tax rates in Argos 8, you will first need to create new locations with tax rates and then assign them to your Printers.
Here's more information on setting up your locations and assigning tax rates to them.
Once your locations have been created and configured, navigate to the Printers tab in Argos Manager.
Highlight the printers that you wish to set a new location for.
Right click on the selected printers and select properties.
Navigate to the Printer Information tab.
Click the dropdown under Location and select your desired location from the list.