I don't know if this helps you with FSX, however documentation for FS2004 modules has always been rare too, so this might be of help.
Here's an example of an FS2004 module: http://pastebin.com/AQG2H0Vc (written by myself, public domain). From my experience the APIs haven't changed that much so with a bit of luck you'll be able to adopt the example for FSX.
Important is the Linkage global variable (type GAUGESLINKAGE) because it includes the MSFS version (0x900 in the example) and callback functions that MSFS shall call. I don't remember exactly what ImportTable was for, however if you're not writing code for gauges, you may just ignore it for now.
Have you checked the SDK that is shipped with the FSX Pro/Deluxe edition?