The X3D spec is still relatively young so the lack of tutorials in general is going to be an issue for a while yet. To date, only the abstract spec has been sent to ISO. The two specifications for the file formats and scripting APIs are not yet formalised. However, to get you going, some VRML97 tutorials and the X3D abstract spec should be enough to get you going.
For the Xj3D-specific part, the loaders are just like a stock loader. There’s a few restrictions and a lot of extra capabilities, but overall, it’s usable just like any other loader. If you want to do in-depth coding using the toolkit as a more general application framework, yes, we are sadly lacking in tutorials. At least some of the issue is that we are not yet completely settled over the architecture. Things are still tossing and turning a bit as we sort out more of the core X3D spec requirements. Writing a tutorial would be OK, but it would be soon out of date. We have a collection of tutorials on extending the codebase to add neew nodes, renderers etc, but nothing about using it in an application environment yet.