Install CubicChunks first: https://minecraft.curseforge.com/projects/opencubicchunks
Dungeon generator will generate dungeons according <world_save_folder>/data/labyrinth/labyrinth_level_preset.json file.
Download example at Link Removed: https://www.mediafire.com/file/rer6eipsras3xe7/labyrinth_level_preset.json/file
"minX", "maxX", "minY", "maxY", "minZ", "maxZ" define area in cube size units, where generator will replace cubes. To convert coordinates to block units, multiply them to 16.
"generator_type" define a set of cube shapes and rules of placing. There is 4 types: "basic", "claustrophobic", "village" and "lava".
"mapping" define blocks which will be used to place shapes in world. "walls" for walls, "floor" for floor etc.
You can peek a list of keys here: https://github.com/Foghrye4/Labyrinth/blob/MC_1.12/src/main/java/labyrinth/worldgen/LevelsStorage.java#L34
You don't need to install this mod at client side while connecting to server with this mod installed.
Note that "Labyrinth" is not actually a labyrinth. It just randomly generate continuous walls next to each other, so it is possible to find completely sealed rooms or huge open spaces.