Do you mean you want people to register first in order to have access to the forum?
If so, log-in and access 'ADMIN OPTIONS'. Go to 'MAIN FORUM CONFIGURATION' and make sure the option to require registration is marked as 'yes' and not 'no' then click 'SUBMIT NEW CONFIG'.
I hope this is what you meant instead of keeping the forum open while just making a certain area psssword accessible.