Please install this: http://www.microsoft.com/download/en/details.aspx?id=35
This fix can also be required even when this error message is not shown by Windows (this particularly seems to happen in Win8).
Check for d3dx9_41.dll and d3dx9_43.dll in your C:\windows\system32\ directory.
The symptom is that the Launcher lists no DLCs or Mods, and if you hit "Play" the game does not start. Can often be the case after installing the game on a new machine or after a Windows re-install.