From Minetest Wiki
Things you'll have to consider
The DirectX export script is not enabled by default on Linux, you will have to enable it.
- Make sure your mesh is child of Armature.
- Make sure there's only one bone parenting all others (having a child bone being parent of another one is no problem).
- All faces need to be members of a vertex group.
- Make sure all of your vertex groups have same name as at least one bone.
- Don't forget to export animation when exporting to .x file.