In each Entry it must be marked "Taxable". That can be "defaulted" in each Client for Time and/or Expense entries. It will only "mark" it when the entry is created. It will NOT retroactively change them.
You must have a Tax setup in the Taxes database. That can also be defaulted in the Client record so that when you create a new invoice, it will add that tax to that Invoice. Again, you will have to manually add it to the invoice if it is not there.
Once those things are done, it will have tax.
Please keep in mind that you can download the same PDF documentation that is built-in to the app. Go to the bottom of the Time Master page on our website and you can download it.