On the PC with Vegas I have DivX 6.2.5 installed. No problems with that one (I believe it is still the latest version). When I have to render to DivX, I start with the default - uncompressed - AVI profile, click "Custom", go to the "Video" tab then click the "Video Format" drop down list an DivX should be there. You can then click the "Configure" button and bring up its options screen.
If it's not there, you'll have to check the CODEC is correctly installed on your machine. Also bear in mind that it's a commercial product, so while decoding is free, encoding is not.
The previous poster is right about not being able to open DivX in Vegas (not entirely sure why) so you'd need to create intermediaries in VirtualDub or similar.