The FSImp message window displays errors and warnings when they arise. Here is a list of possible messages.

ERROR: System has no high resolution timer
The plugin needs a high resolution timer. Your computer system CPU usually has one built in.

ERROR: Folder structure! Plugin badly installed ?
The plugin needs to be installed correctly. Check that all the files were unzipped into the correct place WITH folder information into the correct folder.
Another reason may be that another plugin has changed the working folder. This can happen if it uses a common dialog (eg open file) which is allowed to change the current working folder.
See install

ERROR: FSImp does not work with X-Plane version NNN
Some versions of X-Plane and FSImp are incompatible with each other. Please visit the www.fsimp.com website for the latest information on compatability.

ERROR: The setup file 'FSImp.txt' not found
The plugin needs this file for its parameters.
See install files

ERROR: The scenery file 'scenery.cfg' not defined
The plugin needs to know where this file is. It locates ADD-ON FS sceneries. You must specify its location.
See setup

ERROR: The scenery file 'scenery.cfg' not found
You have specified a location for this file but it could not be opened. Maybe the full path name is incorrect.
See setup

WARNING: No areas found in scenery file 'scenery.cfg'
The file was defined and found. However it resulted in no scenery.
Check the setup again.
Delete the file FSImp\srclist.txt and try again.
See setup sceneries

WARNING: No suitable ADD-ON areas found in scenery file 'scenery.cfg'
The file was defined and found. However it resulted in no scenery.
Check the setup again.
Delete the file FSImp\srclist.txt and try again.
See setup sceneries

WARNING: Tiles will not be loaded as long as Sceneries Dialog is open
The Sceneries Dialog is open. FSImp assumes that you are adding some sceneries and so stops loading scenery tiles. When you close the Sceneries Dialog, this message disappears.
See Sceneries Dialog

WARNING: checked option 'Performance->GL Element Lists' may crash this version of X-Plane
If your system crashes, try disabling this FSImp option.
See Performance Dialog

WARNING: checked option 'Performance->GL Quads' may crash this version of X-Plane
If your system crashes, try disabling this FSImp option.
See Performance Dialog

WARNING: If you have problems, set X-Plane 'Settings->Rendering Options->number of aircraft' to 1
If your aircraft falls through solid ground (runways etc) or seems to hit non-existing ground then try setting this X-Plane option to 1. I have not been able to find out why this happens.

FSImp plugin for X-Plane