Your DLL/XML files being corrupted was just one of the possible reason why the Addon Manager won't show up in the menu, and it's now fixed in P3D 4.1 and up so, unless you are still using 4.0, it's likely the cause is something else.
First, be sure you are running the latest Live Update. And, be sure you excluded the whole Addon Manager folder from the antivirus, otherwise the update might not work.