Differently from "extensions", you don't usually install anything when using classes. You just download them and place them somewhere where you can use them.
Here are some easy steps:
- Download Tweener, either from the stable downloads list, or from Subversion. If you're unsure of which version to download, see this page.
- If you downloaded a ZIP archive, extract the files somewhere.
- Move all files, including their folder structure, to your project source directory.
- Use them on your project by importing the class and calling its methods. Make sure you have the class location included on your classpath list.
If you're new to classes, see the how to use a class page. If you're used to MC Tween instead and wants to know hot to adapt to Tweener, also see Differences between MC Tween and Tweener.
Once that's done, good Actionscript editors such as FlashDevelop will provide syntax highlighting, code hinting and auto completion by default. If you're using the Flash IDE, however, you'll need to download the code syntax highlight installer from the download list.