Hi, it appears as if bezier has been typo'd as "beizer", can you try correcting the spelling and seeing if the problem persists?
To address how the Godot addon functions, the gltf_level_register.tscn is run on startup, and iterates over all the Resources defined in the "Extensions to Register" Array. This tells Godot that it should use that Resource, and the Resource itself which provides the functionality and is used to define which folders to look in for(this avoids iterating over all of the files in the project and applying the addon's importer only to the files we want).
Note that the curve importing will only apply to gltf files in the Resource's Level folder, which in the demo is res://data/levels
