To apply a tax to a client, you have to define it at the client level (People > Clients). To add/edit a tax, click on Edit client.
Note that you can define more than one tax for every client. You can also define negative taxes by simply adding a "-" (minus) before the number.
These taxes will then automatically be taken into account in your invoices/quotes for this client.
You can also edit the taxes directly in the document by clicking on Actions > Edit details.