You have probably put nadeoimporter.exe and NadeoImporterMaterialLib.txt in the wrong directory.
You need to put nadeoimporter.exe and NadeoImporterMaterialLib.txt inside ManiaPlanet installation directory, which usually is : C:\Program Files\ManiaPlanet or C:\Program Files(x86)\ManiaPlanet.
And then inside Convert Assistant go in Options->Change importer location and locate the NadeoImporter again.
It is no problem, i am willing to help with any error i can reproduce, but that i can not make that one .
It looks like something is wrong with MeshParams xml file, can you post template you use, or if you do not use template, post your MeshParams.xml file.
I really do not know what are fbm folders, can you explain?
There are few folders where you put your templates: xmlItemTemplates, meshParamTemplates, fontTemplates, and reports folder is for last report from CA. Other than that there should not be any other folders.