Run
GSpot and load your DivX file. It will tell you:
1) Which codecs your file needs
2) Which (if any) of those codecs you have installed
I'm assuming you already have the AC3Filter codec installed, but if not, go to
this page. Download it from there and then go down to the bottom of the page where it says "Step 3". Read that for information on how to configure it properly.