Took me some time to figure this one out.
If you want to install an mcp module it could be done through the acp. Modules are placed correctly and everythoing shows correctly inside system->Moderator Control Panel
Front of the board doesn't show the added modules and when searching the db i saw that there wasn't any auth given to these modules.
So i added the acl_m as authentication which gave me an error updating the module. The reason is obvious when reading the code:
In acp_modules @line 557 it reads:
$directory = $phpbb_root_path . 'includes/' . $module_class . '/info/';
$modules_class is mcp in this partiicular example so it is looking for the info file inside the "standard" info install directory of modules....
I cant think of a simple solution other than re-write this function OR create a similar function where the info files of modules from extensions can be read...