My FSX keeps telling me to update the Addon Manager, but the latest (3.0.015) is already installed!
Then it means you haven't. Download and install the latest installer and install it.
Also, don't confuse the Live Update notification, which is only telling you to update the Addon Manager, because it *knows* your version is outdated, with the ERROR message, which happens whenever there's an error, suggesting to download the latest Stand-Alone Addon Manager.
Does that message looked like an error message ? If yes, it should give you the option to look at the error log, which contains the information we need to understand why you had the error.
If, instead, your message looks like a normal Live Update notification, just download and install the CURRENT Stand-Alone Addon Manager.