Cause: The file is corrupt or misnamed.
Fix: Rename it to Curviloft.rbz (remove "66"). If still fails, use the official 1.8d from SketchUcation archives (requires free account).
Cause: Curviloft produces a "naked" mesh (not a solid).
Fix: Use SolidSolver (another Fredo6 tool) or ThomThom's CleanUp to stitch the mesh. curviloft plugin sketchup 2015 66
Because this is legacy software, you cannot use the Extension Warehouse (it barely existed in 2015). You must install manually. Cause: The file is corrupt or misnamed