The master volume is the slider across the top of the screen in Box Of Stops.
This controls the volume of all the [[Stop|stops]] on the organ. In fact it sets the maximum volume any stop can play at, and combines with the individual volume for the [[Stop]]. Please see the page on [[Combining volumes]] for some more precise detail on how this works.
To find more about how to control the master volume, for example with a volume pedal, please see the section on [[Registration#Volume pedals|volume pedals]] on the main [[Registration]] page.