The Software Development Kit (SDK) is a set of development tools which allow you to develop, create or modify content for the simulation.
SDKs are available for each of the versions 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.
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
You can search within an open page,
by clicking on the page, then pressing CTRL+F.