No that's it, you open it with Mender, then save it, it should come up with a pop up to choose animation ranges, just accept it without changing anything, then wait, it should work.
That's always worked in the past for me, if it doesn't work then it might be a mesh construction error.
Try opening the model in DirectX 9's MeshView (attached to this post) then saving it, that usually fixes most mesh construction errors.
(don't forget that when you mess with the .x file you will need to delete the .dbo file that gameguru generates, and place it in gameguru from a fresh load or you will just see a cached model and not the changes)
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