Ok I have discovered what the problem is. When the Twin Otter installs it puts the panel and sound files into the wrong folder for the panel and sound aliases.
The installer creates a folder in fs9 called "fsfscov" and puts the sound and panel info in that folder (the folder is at C:\Program Files\Microsoft Games\Flight Simulator 9\Aircraft, assuming you installed FS9 to it's default location).
To fix this you need to copy the contents of the fsfscov folder (2 folders one named dhc6_310 (the panel) and one named prosnd22 (sounds)) to the fsfsconv folder, which is also located in C:\Program Files\Microsoft Games\Flight Simulator 9\Aircraft. This will line up the location of the panel and sound with the respective aliases for the DHC-6.
If I have time I will try to rebuild the installer so it works correctly......sorry for the confusion.
EHAM-YSSY B772LR