How Install Beamer Themes in OS X?
The standard "complete" themes like CambridgeUS and others, which are listed in the beameruserguide, should have been installed together with the beamer package and can be use as described in the manual:

\documentclass{beamer}
\usetheme{CambridgeUS}


outer themes are just a subset of the whole beamer theming approach (e.g. infolines).

With regard to installing other custom themes: simply put them into a sub-folder of your local texmf tree under ~/Library/texmf/.