Hi Zolf,
I'm a bit confused - why are we trying to add users and setup a Primary Domain Controller? Maybe I missed something, is the original problem resolved? I.E, is your Zentyal machine able to surf the web (or ping google.com)? If not, I would suggest fixing this before setting up the PDC. Also, is your public IP address assigned dynamically, or is it static? I should imagine it is dynamic but please confirm.
Cheers,
Andy