I believe in the beginning wxSmith simply did not add the resource file to the project. It is added in a "hidden" way as an extension node in the project file. The 3 files are either added manually or meanwhile wxSmith adds them to the project as a resource file, too once you create the resource (I am not sure about this).
However - be care fuel here, as these resource files are not meant to be edited by hand, it might be better not to have them as resource files in the project at all, actually...