Please note that HostelSystem has plans to improve the way in which the taxes could be managed in the system. New functions will probably be released in the new version 2014.
However, until that time it is still possible to  to add taxes to the reservations, with the use of the "Coupons" option

First you should create new coupon by following this INSTRUCTION, the coupon should be named after the Tax, for example "Tourist Tax"

When you will already have the new coupon, you should adjust its settings properly.

If your tax has a form of a fixed value, then it is impossible to add it right now, unless you will manually change the balance of every reservation. It will be possible to add only the taxes which are expressed as the percentages of the values of the bookings.

You should mark the option "% Discounted" and enter the value of the tax according to the suggestions next to the field where the price should be entered.

Please note that if you will put there the value lower tan 100% the system will subtract this value from the whole price, for example:

if you will enter 10% here, the price of the whole booking will be 100%- 10%= 90%
It will mean that you have added the 10% discount to your booking

If you will enter 102% here, the price of the whole booking will be 102%
It will mean that 2% of the value of the booking was added to it.

There is also possibility to set up a flat amount of the tax

If you will enter 10 here, price of every booking with this coupon will be raised by 10.

If you will enter -10 here, price of every booking with this coupon will be reduced by 10.

You can also decide to what number of nights in the bookings your tax should be added

Please have a look at this INSTRUCTION to see how to apply the coupons to your bookings and how they change the price of the reservation.