Let me clarify:
1) There is no function TemporalSmoother in AviSynth v2.5x nor in v2.0x.
2) Using mpeg2dec.dll for v2.0x there's a TemporalSmoother available:
http://users.win.be/dividee/avisynth.html.
3) Using mpeg2dec.dll for v2.5x (which outputs to YUY2, see
www.avisynth.org) I think the same TemporalSmoother available. However I'm not entirely sure whether Donald also included those functions. So you have to try it out whether there is a function TemporalSmoother in mpeg2dec.dll for v2.5x.