Add these to your profile environment.
PLAN9=%%PREFIX%%/plan9; export PLAN9
PATH=$PATH:$PLAN9/bin; export PATH

This port install a default plumbing file.
Just cd to the ports directory and type (not as root)

make install-user

BUGS
This port can cause some problems if you install certain
ports:
- www/mozilla need archivers/zip to build. If zip
is not installed before you build it, this port will
detect $PLAN9/bin/zip and try to use it unsuccessfully
because they do not share the same options.

- x11/kdebase3 build crash in a fish.cpp file if you try
to build it with $PLAN9.
I suggest you to unset $PLAN9 before building any other ports
if you don't want to fall into these problems and reset it after.

- venti/fmt* commands work only with normal files.