Django django-extensions commands unavailable ( graph_models )

I'm trying to install django-extensions + graphviz + pygraph but i can't. I have done the following steps ( under Ubuntu ):

sudo apt-get install graphviz libgraphviz-dev graphviz-dev python-pygraphviz

in the project virtualenv (running python 2.7.2+):

source <path to virtualenv>/bin/activate
pip install django django-extensions

if i run

which python

it selects the python in my virtualenv, so the python i'm using is the right one. in the virtualenv's site-package i have pygraphviz and django-extensions

python manage.py shell
import django_extensions
import pygraphviz 
RUNS OK

in my django project i have added 'django_extensions' in my INSTALLED_APPS

But when i run

python manage.py help

i can't see the commands and they are unavailable.

python manage.py graph_models -a -g -o model.png
Unknown command: 'graph_models'
Type 'manage.py help' for usage.

How can I fix this ? Thanks!