I spent several days trying to get mine to work. A few tips as to how i did it.
1) Firstly, i had to make sure avisynth/mencoder was set as my Priority 1 video engine. (Use the up arrow to move it to the top so its bold)
2) Secondly, i had to setup Haali media splitter correctly. (Make sure Autoload vsfilter is set to yes & make sure your language and subtitle prefs are correct) You also need to make sure vsfilter is installed for AviSynth. Make sure you have a vsfilter.dll file in your c:\program files(x86)\avisynth 2.5\plugins folder. If you don't, you can copy it out of your K-Lite codec pack folder. (I forget where it is at the moment)
3) Lastly, i had to set my speaker output config in ffdshow audio decoder (Under the mixer section) to 3/0/2 - 5 channels LFE. (Check the box) I have a feeling that this has nothing to do with it, but i don't feel like experimenting with my setup until i finish the series i'm currently watching is over. lol
Thats what fixed it for me.
My setup is as follows:
Windows 7 x64 RTM
Core 2 duo w/ 4gb of ram
CoreAVC w/ K-Lite Codec Pack (newest, non-64bit version. 32bit works much better)