This version is OUTDATED and work only with downgraded STEAM 5023 [1.5 - 05.12.2017].
(To play workshop version of mod you should downgrade game from beta branches in the steam)
Actual version (2.0) (Work on ANY version of game)
The new version has its own new build and will be available here: Happy Night 2.0 Final Director's Cut

Steam:
Drop folder 'POSTALXX' inside 'POSTAL2Complete' folder. Launch 'Launcher.exe' inside 'POSTALXX\HappyNight' folder.

GOG:
Drop folder 'POSTALXX' inside 'Postal 2' folder. Launch 'Launcher.exe' inside 'POSTALXX\HappyNight' folder.

Also you can install custom version of old 1.5 build:
Happy Night 1.5 Delete Edition (OUTDATED)