the file formats mplayer are/can be patented in some countries, and its legal status is uncertain in several countries. Because of this, mplayer packages aren't build.
From the source of pkgsrc :
file: ...multimedia/mplayer-share/Makefile.common
RESTRICTED= Prohibited by USAs DMCA and patent law
NO_BIN_ON_CDROM= ${RESTRICTED}
NO_BIN_ON_FTP= ${RESTRICTED}
NO_SRC_ON_FTP= ${RESTRICTED}
......
You can build mplayer and other related mplayer packages building yourself from pkgsrc:
http://cdn.netbsd.org/pub/pkgsrc/stable/pkgsrc.tar.bz2
Uncompress it under /usr.
# cd /usr/pkgsrc/multimedia/mplayer
# make install