Your problem is that you're trying to change the most complex SceniX example without understanding how it works. (Changing the miss program on the preview scene isn't doing anything to the four main viewports, but to the Material Editor preview scene, which BTW you can also switch to ray tracing with the right click context menu. What you really would have to do is to change the RTFxSceneAttribute of the loaded scene.)

Please take a step back and do the following:
1.) Start with a smaller example.
2.) Please use the QtMinimal example which is also able to render ray traced when starting it with the command line QtMinimal.exe --raytracing. Try that to see the difference.
3.) Now in that example there is either a SimpleScene or a loaded scene used (--filename command to load a scene).
4.) After the scene has been defined in that example (e.g. before the line RenderContextGLFormat format;), add your RTFxSceneAttribute with your miss program.
Since you're using the built-in ray tracing programs for everything else in that case, you must make sure that your radiance per ray data is matching the one described inside the appendix of the Using RTFx.pdf exactly or you will get a validation failure from OptiX.
(You might actually need to specify one for raytype 1 in that case as well which would be the shadow ray in SceniX. An empty one will do. There is some gotcha with replacing only parts of the built-in pipeline's programs and not matching the number of ray types. The QtAmbientOcclusion and QtIconTracer examples exchange everything except the intersection and bounding box programs and don't run into that.)
5.) Run the QtMinimal --raytracing until you have your miss program running which does something different than returning the float4 clearColor.
6.) Now go back to the SceniX Viewer and debug into the code which is executed in Menu -> File -> Load. The scene is loaded in the SceneSerializer::load() function, look for nvutil::loadScene(). That is where you might want to change your scene.