BuddyPress.org

You need to log into WPMU’s backend as site admin ( /wp-login.php), and then go to

Site Admin > Options > Allow new registrations

and set the options you want.

Also I am unable to create an extended profile like shown in some of the demos. It only allows me change the “base” and avatar. I can’t put birthday, location, etc.

To add additional profile fields, you need to navigate to the BuddyPress submenu grouping in WPMU’s backend. If BuddyPress is installed and activated, this submenu group is found at the very bottom of the menu column.

So, go to

BuddyPress > Profile Field Setup

and set up the additional fields you want.