it means that you made the d2v file with dvd2avi and are trying to use dgdecode.dll
less likely but still possible is DGindex/dgdecode version incompatibility.
get the latest version from
here (DGIndex and DGDecode)
when you put dgdecode.dll in the avisynth plugins folder be sure to remove any stray mpeg2dec3.dll files that might be hangin around
if that doesn't work then go through the video, audio, tools, options, whatever is up there and post the settings that DGIndex is using