Master couplers are well-described in the HW manual - see pg 100.
Essentially they provide couplers directly via the HW system in addition to those provided by thes set presently loaded. They are accessible via one of the large control panels (View menu). On the panel the couplers which are unavailable are greyed out (you can't couple divisions which don't exist in the sample set. Like any other HW control they can be auto detected to operate with any midi or computer keyboard control. The sample set provider can disable the master couplers if they don't want them used with their sample set.
I don't know the sample set you are trying to use but from the description on the website http://www.sonusparadisi.cz/en/organs/czech/kdousov.html the specification advises using the HW Master Couplers for any couplers required. It would be up to the user to decide which controls on their console (or perhaps on the piston bar) to use for the couplers and indeed what couplers to implement.
Iain