There is nothing you can do to enable Horn Master. The firmware as built either supports Horn Master or it does not. All of the Q3 diesel firmware starting with version 9.3.0 support Horn Master.
For firmware that is built to support horn master, you select a horn master horn (which "activates" horn master) using CV55.1.1 Horn Type. All of the types from 10 on are pre-defined horn master horns. If you select type 2 User-defined Horn, you can use CV130 CV's to choose the number of chimes and specify the pitch of each chime.
>>> Is there a steam version of Horn Master? <<<
A whistle version of horn master is still under development. Don't expect this soon. Whistle sounds are much more complex than horn sounds.