I can see the following line in the log :

including configuration file /usr/local/etc/raddb/experimental.conf

And I left it has it was, so in it, I have :

         python {
                mod_instantiate = radiusd_test
                func_instantiate = instantiate

                mod_authorize = radiusd_test
                func_authorize = authorize

                mod_accounting = radiusd_test
                func_accounting = accounting

                mod_preproxy = radiusd_test
                func_preproxy = preproxy

                mod_postproxy = radiusd_test
                func_postproxy = postproxy

                mod_postauth = radiusd_test
                func_postauth = postauth

                mod_detach = radiusd_test
                func_detach = detach
        }

Now, I restarted the server and I have the following. Note that to make sure I was in the right section, I added the following lines in the "post-auth" section of the "site-enabled/default" file :
The echo_NAS_Info is a test I made copying the echo module under the "modules" folder

echo_NAS_Info
python

The result :

Module: Checking post-auth {...} for more modules to load
Module: Instantiating echo_NAS_Info
exec echo_NAS_Info {
    wait = yes
    program = "/bin/echo %{User-Name}, %{NAS-IP-Address}, %{NAS-Port} "
    input_pairs = "request"
    output_pairs = "reply"
    shell_escape = yes
}
/usr/local/etc/raddb/sites-enabled/default[422]: Failed to find module "python".
/usr/local/etc/raddb/sites-enabled/default[380]: Errors parsing post-auth section.
}

Now it looks like I have a configuration error and not a python module error. Do I have to add more than
I replaced python with example in the default file and I have the same result for "example"
I verified my /usr/local/lib/ and I have the :

rlm_example* files
rlm_python* files

Seeing that echo_NAS_Info loads, i suspect there is something else that has to be done in order to activate de experimental modules/config.
When you say, simply edit experimental.conf, should I change something in it, or by default it should be ok ? The only thing I did is modifiy the radiusd.conf file to add the include line. The python section of the experimental.conf file is uncommented by default.