To change Master volume you need to use the global command "FCxx". xx is a Hexadecimal value from 00 to FF, with FF is the louder.
The same command is used to control volume of all wire that connect machine between them , including to the Master.
However to change the volume of a machine you must used specific machine command or , like for VSTi, tweak command.
More info on the command "FC" and other machine command can be found in the help file "Tweaking and Command".
If you give greater detail of what you want to acheive I can provide you more detail information.