You can download the DivX codec here.

A few words of caution:

There are three editions of the DivX codec. DivX 5 (free), DivX 5 Pro (free), and DivX 5 Pro ($30). The Pro versions have additional encoding features for advanced users, but unless you really like to fool around with compression options, the regular version should be sufficient. The free version of Pro is bundled with adware, unlike the others.

I usually get better results by first creating uncompressed video in Poser, then editing and compressing the video with a third-party utility afterward. It gives me more control over the process. VirtualDub can compress or recompress to most AVI codecs.