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.