Thanks! Found it, used the default MB-339 option.tmc and now get the new skins. Just a slight problem, they are all called "Peruvian Air Force" in the aircraft selection menu (even though the desired DLC skin is shown). A minor irritation, but curious if there is an easy way to fix.
Found the easy fix, just had to edit the option.tmc file to change the name. Obvious, of course...
All is well now.