If you have the latest version P3D V3, you should NOT add the Addon Manager to the DLL.XML file under %PROGRAMDATA% or %APPDATA%, because the file is already launched by the add-on.xml file in the Documents folder.
If, instead, you have an earlier version of P3D V3, which still use the DLL.XML file, the installer would add that line automatically, so there shouldn't be any need to add it manually.