I don't see any option or dependency in the latest Makefile for dvdcss or dvdnav, I don't have a desktop FreeBSD system anymore so I can't try it right now, but I'm pretty sure it worked at least ~1 year ago, I used to use my FreeBSD machine to play DVD's (Before I bought a standalone DVD player) and I'm pretty sure mplayer used libdvdcss for that.
You could try looking trough the CVS logs and see what has changed:
http://www.freebsd.org/cgi/cvsweb.cg...media/mplayer/
Also make sure your ports tree is up to date and you have the latest versions of mplayer and dependencies installed.
As a sidenote, you should use libdvdnav-mplayer instead of libdvdnav, this should be the default version now.