plpython on postgresql 9.1

Hello,

I have installed postgresql on a mac using the 'one click' installer (postgresql-9.1.1-1-osx.dmg). I then tried to import the python language but I get the error:

create language plpythonu;
ERROR:  could not access file "$libdir/plpython2": No such file or directory

Can you help me in figuring out what is wrong? (I.e. where can I get plpython2?)
These are some settings that might be relevant:

Mac OS X 10.6.8

PostgreSQL 9.1.1
python 2.6.1

pg_config --pkglibdir
/Library/PostgreSQL/9.1/lib/postgresql

pg_config --libdir
/Library/PostgreSQL/9.1/lib

Many thanks!

Dario