Do you have the Couatl menu ? You don't say which P3D version you are referring to but, assuming it's V4, the Addon Manager use the 4.1 version and, it's not possible for a 3rd party addon to "downgrade" it, since it's inside the sim so, as long as you have at least 4.1, it's not possible the Addon Manager won't run.
Instead, the Couatl program, that is also required for our software to work, it's a 32 bit Simconnect FSX/SP2 client, so it's able to connect to every version of FSX and P3D, assuming the FSX Simconnect client is installed in the sim.
Our installer of course starts the Simconnect client installer you have in SDK folder of the sim but, if another 3rd party product has installed the wrong/outdated version, it's POSSIBLE it might caused issues to the proper version.
Usually, the right way to fix this, is to start Windows in SAFE mode, and remove all the Simconnect folders from the Windows\WinSxS folder, then restart Windows, and reinstall ONLY the CORRECT Simconnect client, the Simconnect.MSI XPACK installer from the SDK/Legacy folder under the P3D installation folder.