If you use P3D 3.3, that's the cause of the problem.
Although the LM SDK documentation says that starting with 3.3 the new installation system is fully supported, it seems there's still a bug with it, which has been fixed in 3.4, that a file which is crucial for the Addon Manager to find the add-ons installed with the add-on.xml system (the scenery_add-ons.xml file), is not created.
You must update to 3.4 to fix this.