Making missions for Flight Simulator X using the "Object Placement Tool" included in the Software Development Kit is easier than editing XML by hand, but we're the first to admit that it's not an inuitive tool to use. Some good news: a more robust third-party mission editor was just released, and it makes mission development and testing a lot easier for developers.
"FSX Mission Editor" was created by Jim Keir and is published by FSAddon Publishing. Some of the most useful features of the tool are a visual interface, full error checking, and synthetic voices for creating placeholder dialog. There are three versions available: freeware, payware (with even more features), and a version licensed for the creation of missions to be sold commercially.
Whether you already devleop missions for FSX, or have been curious but hesitant to get started, the process just got a whole lot easier!
You can learn more about "FSX Mission Editor" and download it at SimMarket.