First you need to setup your Tax Rates, to do that you need to go to:

• Menu > Company Settings > Financial Tab > Add Tax Rate.

After you are done with that, go to:

• Menu > Invoices > Details Tab > Edit > Item Line you wish to Tax > Click Item Line edit > Check mark the box (Collect Tax) > Save & Close.

NOTE: To add Tax to the Invoice the status must be as Open.