You have to have your agent adjust your booking to include pre-paid gratuities. This is normally done when you make the booking.
Really, at this point it would be simpler for you to just go to the guest relations desk once aboard the ship and arrange to have the tips placed on your account. They usually have a stack of forms out for this purpose. Just fill it out and drop it in the nearby bin. No need to stand in line.
As with pre-paying (which is technically different than having them added to your shipboard account) you still get the vouchers and envelopes delivered to your cabin on the last day.