As pointed out by BoggyB, yes the library only contains a header file. So there actually is no support for J2B at all.
I've pointed this out to the author plenty of times, but as nowadays the library (and software) is defunct it's not going to get updated anymore. And shamefully all the descriptions aren't getting updated either.
So whatever thing you see claiming to be able to play J2B-files and is based on libmodplug, ignore it. It doesn't work.

And yes, I -have- looked at the code and I couldn't fix it either. :P maybe someone else here wants to give it a go?