This because you haven't installed GSX, which is required to have Docking System.
When the Zurich manual was written, we used to include Docking Systems in each scenery but, after the release of GSX, this became very inefficient, because if we kept doing that, we would introduced multiple copies of the same object if you had several airports with the same docking systems so, after GSX came out, all our Docking Systems are coming from GSX, so you install it once, and there will be only one copy of each Docking System model, regardless how many airports you have that use it.
GSX is of course completely free to use at all FSDT airports, so you just download it and install it: a purchase is required only if you want to use it at non-FSDT airports.
