If you want to compile your own dll or pull the code you can do so from here.
Otherwise head over to the Sitecore Marketplace to download a Sitecore package or it is available on NuGet to install directly into a solution. The module should work with versions 8.0 and above.
You will need to generate a API key for the Computer Vision API and add the key into 'AutoAltTags.Config' for the module to work. An alt tag will be generated upon uploading or saving a media item. If an alt tag is already set it won't be overridden.
More information can be found in this blog post.