Winamp is a 32-bit program, therefore it will only use the 32-bit version of
ffdshow.
ie. you need to install the 32-bit version of ffdshow, not the 64-bit version.
The problem could be with registering the splitter...
Save/extract the mp4splitter.ax file to somewhere like:
C:\DirectShow\mp4splitter.ax
Go to: Start -> Programs -> Accessories:
Right-click "Command Prompt" and select "Run as administrator"
In the command prompt, type the following:
regsvr32 "c:\directshow\mp4splitter.ax"
Hit Enter
Then go to: Start -> Programs -> ffdshow -> Video decoder config:
On the Codecs page, make sure all of H.264/AVC, H.263, DivX, Xvid, Other MPEG4 are all set to "libavcodec".
If not, click where it says "disabled" and change it via the drop-down menu.
Click Apply/OK.
Then go to: Start -> Programs -> ffdshow -> Audio decoder config:
On the Codecs page, make sure AAC is set to "libfaad2".
You shouldn't need to set MP3 to "libmad", but can if you wish...
Voila! :-)