Okay, well, obviously, make sure you exported again. I know this sounds like a "calling me stupid?" thing to say, but I have done this a number of times and then gotten furious at my FS for not working right.
What's this about changing the opacity for the texture?

Do you mean editing the actual texture so it is transparent before you put it on the aircraft? If that is what you are talking about, forget it. That isn't something that you can or need to do.
On any FS aircraft, the transparency of an object comes from the object being made that way in the program it was created with.
And I guess you're used to speaking French instead of English, right?