The textures on the FSX version of the Twin Otter package from the download section contain some errors and don't display correctly. I've found an updated model of the aircraft and applied the existing textures to it. Everything seems to display as it should now.
The file is hosted on Avsim.com. You will need a (free) account with them to download this file. Head on over and sign up!
http://library.avsim.net/download.php?DLID=169025Enjoy!