Hi,
In FMS DM go to settings and set the XP12 path to the X-Plane 12 textbox as per the below image.
FMS DM → settings for Simulator Path Textbox is also supporting for manual change in the path. so, you can also type the correct path.
NOTE: In your case, you have to set the path folder where the X-Plane.app application file is located.
After setting the correct path just save the settings and rescan the addons from the Addon Mapping button
Please feel free to inform us about any issues with the Navigraph Product.
Happy Flight 
Kind Regards,
Vishal Ahir
Navigraph