Anyone using the MacPorts version of PostgreSQL to run xTuple will run into difficulties installing the PL/v8 extension. This fix works for me.
Follow the instructions at https://github.com/xtuple/xtuple/wiki/Installing-PLv8 for Mac OS X.
Before running the "install_plv8.sh" script, edit the script and find the line that reads:
PATH="$PGDIR/bin:$PATH"
Change this line to read:
PATH="$PGDIR/lib/postgresql93/bin:$PATH"
(You'll want to change the exact path to match your installed version of PostgreSQL. Here it's 9.3.x.)
Save & exit, then execute the script. When prompted for the PostgreSQL installation path, enter "/opt/local" and everything should work properly.