Hi,
There is documentation on how to use ladspa with the fluidsynth shell commands here:
It explains the concepts behind die API, and the shell commands are nearly transparent mappings onto the API. So I hope that document also helps in your case.
Please let me know if there is anything unclear or missing from the LADSPA docs. I wrote them quite a long time ago but have not had much feedback on it until now.
Cheers
Marcus