Make sure you are downloading X-Plane 9 aircraft; x-plane.org has a number of X-Plane 10 and X-Plane 9 software, but in some instances, they are not compatible with both. Also, make sure you extracting the downloaded files and installing them correctly.
I don't know if there is a way to stop the message from displaying because it is telling you that there is an error, which is important. With or without the message, there is still an error. :-)