From reading the page it seems like you download the files at the bottom of the page which you linked (the 2 folders Behaviors and Unitysteer and the license.txt). All the individual scripts seem to be there.
You can also download these scripts as a zip file by clicking here. But again, it's the same files which are located in the folders at the bottom.
As for documentation, looking at the examples might be useful (link to examples)
answered Feb 15 '10 at 08:51 PM
The latest version of UnitySteer is hosted at the github site you link to. I periodically upload updates to the site, but you can consider github the most up to date version.
As Sebas has already linked to the projects, you may want to start reading up on git itself:
Bear in mind that as a test area for steering and vehicles, UnitySteer is very much a work in progress. Code contributions are welcome, as are issue reports.
answered Feb 15 '10 at 09:25 PM
Its also now available in the Unity Asset Store under Scripting Packages.
answered Dec 14 '10 at 02:44 PM