Look in DefaultFurnitureCatalog how the subclass [font=courier new]ResourceURLContent[/font] is used.
But as [font=courier new]lightSource.obj[/font] may be in different places (in a file during development, then in [font=courier new]Furniture.jar[/font] or [font=courier new]SweetHome3D-x.y.jar[/font] during deployment and finally in a SH3D file when saved), I would suggest to use a copy of that file in your plug-in.