Hello William,
If you would like to get the codecs, you can build it easily yourself by following instructions here for compiling ffmpeg [1]. However, if you just want to try playing back WebM/VP9 files, you can install the latest VLC[2] as the easiest way to test it out.
Finally, if you would like to encode videos in WebM/VP9, here are instructions for encode settings in ffmpeg[3]. And if you have more question, you can always ask it on webm-discuss@webmproject.org[4] mailing list. THanks.
[1]: https://trac.ffmpeg.org/wiki/CompilationGuide/MacOSX
[2]: http://www.videolan.org/vlc/download-macosx.html
[3]: http://wiki.webmproject.org/ffmpeg/vp9-encoding-guide
[4]: https://groups.google.com/a/webmproject.org/forum/#!forum/webm-discuss