Hi Plautzer -
You should download filters compatible with Avisynth 2.5.x - the post by shOdan announcing the release of Avisynth 2.5.2 included a link to a site listing most Avisynth filters in 2.0.x and 2.5.x equivalents. You'll also need a new mpeg2dec3.dll that is Avisynth 2.5.x compatible.
Here's the link:
http://www.avisynth.org/~warpenterprises/
What you might do is make a new directory in moviestacker with the 2.5 filters, then point moviestacker to the new directory. However, moviestacker is currently not totally compatible with Avisynth 2.5.x - it's easier to copy Kwag's optimal script and change the directories for your filters in the script.
Hope that helps, --Tony K.