Once site studio is active, you can login at:
http://hostgator.com/build or http://69.93.237.199:8080/servlet/psoft.masonry.Builder
Resellers usually want to use the second link because it does not have hostgator.com in it.
The login for site studio will always be the cPanel username and password. If the cPanel password changes, you will need to contact sales@hostgator.com to have the information updated.
Note: Site Studio requires the domain name to be fully propagated and resolving to HostGator.