that's a problem, without anything in there you'll have to use loadplugin() to do anything. It's doable...but autoloading is so much nicer.
If you can do a search for dgdecode.dll and put it (and your other plugins) into the avisynth plugin folder that would be best...but if you can't find it you can get it from
here
if you do end up downloading it you may have to re-index your .vob files (the version of dgindex and dgdecode.dll have to match or it'l give you strange errors)