not entirely sure myself but here goes,
I don't know if mencode supports coreavc or not and not sue if you need anisynth installed.
if you have a codec pack like k lite or cccp then
open ffdshow video decoder > go to codecs > h264 > disable
install coreavc and all you media will now be handeled by coreavc atleast on the PC when using WMP11 etc
i think mencode has it own h264 codecs and the only way to use it would be to have avisynth installed. But i hope i'm wrong about this i would like to use the mencode rather then avisynth but with coreavc to handle my h264.