Most Atlassian products have AUI installed as a plugin. The flat pack is great for prototyping or deploying in apps which can't load a plugin.
The AUI Sandbox is a tool that will help you become acquainted with AUI and give you quick code examples of how to use its components
Serve AUI from CDN
Serving AUI from a CDN will make your pages load faster and you won't have to maintain out of date resources.
Download flat pack
Download the static files and bundle it in your project like any other resources.