Ok. DivX...
Use the link I supplied and it will tell you there what you need to download/install. There are three or four items. The one named "Get It!" is the DivX Doctor. It recommends QT 5 or greater (you have it) and CarbonLib 1.5 or greater. And then there are three other items. I don't know what their specific uses are, but I downloaded them. I followed the installation instructions on each of the page(s). It's been a while.
Anyway, with DivX Doctor, I use it when I have an .avi that has just a white screen w/ audio. It will usually fix the video. I use it when I have an .avi with video and no audio. It's a simple drag-n-drop app. I take the .avi and drag it over the DivX app icon and it will open DivX and fix (most of the time) the file.
There are some preferences you can play with for your needs. There will also be a file in the DivX Doctor folder that links to their forums. You may get some help there too.
I tried a free download of QT 6, but had way too many problems with it. But, I am still using OS 9.1 . That may have had something to do with it. Not sure.
Also, if you use Quicken (I am using 2001 Deluxe) then any CarbonLib higher than 1.2.5 will not work for it (Quicken). I have two Hard Drives installed. I have LimeWire and DivX on one with CarbonLib 1.6. On the other is Quicken with the older CarbonLib extension. I just threw this in, FYI, in case you end up with more problems than you started with.
Well, as you can see, I'm not very technically-literate (Is that a proper term?). But, I get by and I hope some of this helps you.