Only plugins in the Plugins directory of an independent (not MeGUI included) install of Avs+ are auto loaded.
To use any other plugins, need to add eg
LoadPlugin("Z:\MyPath\Mydll.dll") to the script before calling the filter.
LoadPlugin:-
http://avisynth.nl/index.php/Plugins#LoadPlugin
You are better off installing independent Avs+ and using that with plugin autoload.
EDIT: Note, with the current avs+ standard setup, it can install both x86 and x64 avs+ side-by-side.
AVS+:-
https://forum.doom9.org/showthread.php?t=181351
EDIT: removed '
s' from LoadPlugin
s("Z:\MyPath\Mydll.dll")