I think there was a missunderstood in your message.
First you said, you cannot download aircrafts, then you said, you cannot use them inside the game. I guess, your real problem is you cannot use the mentioned airplanes.
These aircrafts are FS2002 compatible models, however all must work in FS2004 too. There can be some animations bug, like you cannot see the engine rotor, or the gears...those can happen, but basically you will be able to fly the airplanes. When FS tells you that aircraft is incompatible, and would you like to load it anyway, simply select YES.
Unfortunately some aircraft models that we use in our fleet, are very old ones, and there was not a newer since yet. We have to live with it, and we trying to renew the fleet according this problem, and modify the aircraft models to have minimum incompatible ones.