To the best of my knowledge there is no plan to build a plugin for Outlook for Mac.
Just to avoid any false impression of deflection, I am Mac OS X, Linux, BSD Unix and Oracle/Sun Solaris user.
The plugin for Outlook 2010 is unlikely to work smoothly for Outlook 2013 due deep changes introduced:
Currently, Outlook 2011 doesn't support any kind of Plug-In architecture. Marketo can't really make something that plugs in to a connection point that doesn't yet exist. There's a thread on Microsoft's site about this: