Please see documentation at....
When using 'custom' search paths of the "type_plugin" style core language packs keep the help in a directory of the form help/type_plugin/. Additional search locations (ie, for third party plugins) require it to be help/plugin (the type_ is dropped). This is inconsistent and confusing.
Obviously, if this is fixed it would break help files in all third party plugins - I don't think there are a huge amount however. I therefore propose this for Moodle 2.0 as loads will change anyway.