Some add-on content built for Flight Simulator X includes executable code built with Microsoft's premier development platform, Visual Studio. If you're using such an add-on, there is an issue with an update to Visual Studio that may prevent the add-on from loading properly.
Visual Studio 2005 Service Pack 1 has been released, and includes updated "redistributable" components. These are files required at runtime by applications built using Visual Studio, and are not backwards compatible in most circumstances. In other words, content created for Flight Simulator X using Visual Studio 2005 SP1 will require the updated redistributable components or it will fail to load. It is the responsibility of the add-on developers to include all components required for their for their content. This issue will not be affected by the forthcoming release of Service Pack 1 for Flight Simulator X. If and when the redistributable components are made available for download separately, we will make them available here.