Try this:
SELECT acs_user__new(null, 'user', now(), null, null, null, 'someusername', 'mailto:someone@example.com'; , null, 'somefirstname' , 'somelastname' , 'somepassword' , 'somesalt' , null, 't', null );
The order of arguments is as follows: user_id,object_type, creation_date, creation_user, creation_ip, authority_id, username, email, url, first_names, last_name, password, salt, screen_name, email_verified_p, context_id