Hello. Your vessel configuration files must be placed within the $ORBITER_ROOT/Config/Vessels folder. If you are attempting to use the scenario editor with an existing vessel, make sure:
1. The configuration file is placed in the "Vessels" folder.
2. The config file does not contain the line "EditorCreate = false".
Vessel configuration files define a vessel's parameters for Orbiter. At the very least, the file requires a class name, and a mesh name (if you are not using a dll module, in which case the module itself could add the appropriate meshes).
For example, take a look at Carina.cfg.