There is an SDK and an Object Placement Tool, that's all you basically need. The SDK is supplied with certain versions, I think Deluxe and above, and is the have all of information and files needed for everything from making missions to gauges and scenery objects. Included in the SDK is the Object Placement Tool .dll. This you will need to link in your CFG file by adding a small bit of code. This tool can then be accessed while in FSX from the tool bar that will allow you to not only place scenery in the FSX sandbox but also build a complete mission by creating a mission folder with the appropriate files inside and creating a XML that includes the code to run the mission.

There is also an inexpensive program to build and edit the mission using a graphical interface called FSXMissionEditor. I use it because it makes building missions easier.

There is also another one that is supposed to make missions for beginners, from start to finish and is called Instant Mission Maker. It works somewhat well from what I hear but is not as easy to edit.

There are sites that are dedicated to just missions like http://www.flightsimulatorxmissions.com/index.php and http://www.fsdeveloper.com/forum/forumdisplay.php?f=59 that might help. They will also have links to sites that have information on SDK installation and such that will come in handy.

Good luck, there is much to learn and it's hints and tips are spread out among many sites and forum posts.

Hope this helps,
Brett