Logged In: NO

This script works.

AviSource("sample.avi")
loadCPlugin("C:\Program Files\AviSynth 2.5\plugins\ffavisynth.dll")
ffdshow("default","isSubtitles=1,subAutoFlnm=0,subFlnm=subtitle.ssa")

Use isSubtitles, not showSubtitles.
Spaces in the option list are not allowed.