The FSX SDK is a separate install you have to manually run from the FSX Deluxe DVD.
It is essential that the SDK match the FSX version you have installed. i.e. If you use FSX SP1, you must have SDK SP1A. If you use FSX SP2, you must have SDK SP2 (Acceleration also uses SDK SP2).
You have to install SDK RTM, update to SP1A and then update to SP2 if needed.
Here is an article that explains the process better than I can - http://www.fsdeveloper.com/wiki/inde...tion_%28FSX%29
@ PawPaw's house - near KADS, Addison, Texas, USA