SDK

Software Development Kits (SDKs) are available for each of the versions of FS.
You should use the appropriate SDK version to match your version of FS.

The SDK is actually a collection of task-oriented SDKs.

There is no single tool that you run, rather it's collection of tools, each of which perform seperate tasks.
Some of these SDK tools will call other SDK tools.

The majority of tools are stand-alone, and launched outside of the sim.
Depending on version, a few tools are launchable from the in-flight menu.

You should use the appropriate SDK version to match your version of FS.

More about the SDK...

FSX

Toolset

SDK Toolset (FSX) The components of each SDK, with links to the documentation describing how to use it.

Installation

SDK Installation (FSX) How to install the SDK for FSX/FSXA.

Documentation

SDK documentation, and access links to some tools, are available from -
Start -> All Programs -> Microsoft Games -> Microsoft Flight Simulator X SDK -> Flight Simulator X SDK

RTM, SP1, & SP1a documentation is supplied in html format, with each folder containing the relevant html file(s).

SP2 documentation is supplied in chm format, as a single compiled html file, fsxsdk.chm.
located by default in C:\Program Files\Microsoft Games\Microsoft Flight Simulator X SDK\SDK

FS2004

Toolset

SDK Toolset (FS9) The components of each SDK, with links to the documentation describing how to use it.

Installation

SDK Installation (FS9) How to install the SDK for FSX/FSXA.

FS2002

Toolset

SDK Toolset (FS8) The components of each SDK, with links to the documentation describing how to use it.

Installation

SDK Installation (FS8) How to install the SDK for FSX/FSXA.