Hi Brambo. It would help to know exactly where to find the offending file. Is this an original design or a repaint? Repaints usually include the modified textures only. If it's for a 3rd party design you will need to install the original aircraft first, then add the new textures as in the defaults. There should be install instructions included with the download. Look for a file named Readme.txt or similar & open it in a text editor.
My little tute is for adding repaints to a default aircraft but should give you the general idea. http://www.simviation.com/lair/Repaint2002.htm