in registry(use regedit)

goto [HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\VisualStudio\7.1\Languages\File
Extensions]
add Key = "*.d" Default Value = "{B2F072B0-ABC1-11D0-9D62-00C04FD9DFD9}" (aka
*.cpp)

One more, if you using Whole Tomatos Visual Assist

goto [HKEY_CURRENT_USER\Software\Whole Tomato\Visual Assist X\VANet] append ".d" in Key "ExtHeader"

and Enjoy!!