Community builder doesnt send you to registration form.

1. create a menu item to something that needs the user to be login to see it
ex from community builder a menu to : edit my profile. - and see if will ask you for registration or just that noy authorized to view and login needed
2. if you have setted in adsmanager submission type: user account required to post ad.
and if you click on a menu that is from adsmanager : Add ad (write ad) - this will send you to registration.

Also on the demo site if you click on Post an ad.. will be redirected to registration form.

A solution for this would be to make the menu item : Post an ad - visible only to registered users in joomla menu settings

Or maybe will be in the future an option in adsmanager to dont send to register page.
Like : submission type: only reg users aloud ... and redirect to login
But here i think will not be possible that after login to redirect you to post an ad menu item... so nothing to gain (because depends if you approve users by email ..or by admin..or etc

So a solution would be to create a menu to login - or the cb login that you use
1- A - use cb login module and set redirect login to post an ad menu item
1- B- use core system plugin to handle register redirect to a subscription component for example

2- like gfoxtrot was saying you may use bt login or SC login which has a nice design. This login modules have settings to work with community builder and you can also redirect to that menu item Post an ad.

Bt login changes his design when working with CB and it's ugly like cb module, so search SC login...

and you can even rename the login button with Post an ad .. an use redirect to Post an ad menu item

that post an ad menu item .. dont put it in main menu... put in a formal menu but the item menu should be published

Look here how SC login looks: