User Registration Page for add on domains

I have a site installed with

WordPress MU 2.9.1.
Buddypress Version 1.1.3
BuddyPress Business Child Theme 1.0.1

http://mysite.org/register works great.

http://addonsite.org/register gives me a 404 error.

What do I need to do in order for the registration page to work in the subdomain?