The ‘new’ tool for FSX developers is 3 ds Max. I believe they started with version 2007 for FS, but am not sure. Meanwhile there’s a version 2010, but older versions still work fine for FSX development.
3DSMax is published by Autodesk, who also bought Discreet’s Gmax only to dump it and get rid of a competitor. 3DSMax costs somewhere around US $ 3,600, so out of reach for most developers in this niche market.
You can purchase 3DSMax here at Autodesk. But most people will buy it ‘locally’.
Autodesk 3DSMax Youtube Learning Center with some great tutorial videos.