Animations are sort of broken (compared to how they used to work), but you have two options:
1. Upload an AVI or similar video file along with your images and CAD files. People will have to look for, and download the file to view it. I believe the advantage to this method is if you do this for ten models (or more), you'll unlock the animation badge... Not a huge deal, but why not do it?
2. Create an animated gif, and upload it with your files. This will be placed along with other images in the gallery. If you load it as the primary image, it will most likely be animated when people see the SMALL version of the image on the main model page. When your page is loaded, the gif does not "auto play" like it used to. Instead, the image must be clicked. It will then animate properly.
Check out my kaleidoscope model as an example. The second image of the gallery is an animated gif.