To enable this compatibility layer move this entire directory "officialcompatibilitylayer" along with it's contents retaining the same structure, either into the .minetest/mods/ directory, or if you want the compatibility layer to ALWAYS be enabled then move to under .minetest/games/clonecraftlibre/ only if you really want it always enabled.

if you move this to .minetest/mods/ then you can select individually which worlds use the compatibility layer and which parts of it.

The compatibility layer is not enabled by default because that would deviate too much from the 1.8 experience of The OG Game which does not have many of the items added by the compatibility layer.
