Hello,
It needs a database username/password.
DB users are only added after you create a database.
Without a database, nothing is added yet, so you can't get in.
Once you create a database+db user, you can use the db user, but you can also use your DA login/password.
For root level access, you can grab the da_admin password from:
/usr/local/directadmin/conf/directadmin/mysql.conf
This login/pass should work in all cases, before db's are added.
John