I have this working (I think I've got everythign you need to change in bold
_config.php
Authenticator::register_authenticator('ExternalAuthenticator');
Authenticator::set_default_authenticator('ExternalAuthenticator');
ExternalAuthenticator::setAuthDebug('/tmp/sstripe_debug.log');
ExternalAuthenticator::createSource('domain','LDAP','User Directory');
ExternalAuthenticator::setAnchorDesc('User ID');
ExternalAuthenticator::setAuthSSLock('domain',false);
ExternalAuthenticator::setAuthServer('domain',fqdn.of.domain.controller);
ExternalAuthenticator::setOption('domain', 'basedn', 'ou=SomeOU,dc=fabrikam,dc=com');
ExternalAuthenticator::setOption('domain', 'attribute', 'sAMAccountName');
ExternalAuthenticator::setAutoAdd('domain', 'Default Domain');
ExternalAuthenticator::setOption('domain', 'firstname_attr', 'givenName');
ExternalAuthenticator::setOption('domain', 'surname_attr', 'sn');
ExternalAuthenticator::setOption('domain', 'email_attr', 'mail');
ExternalAuthenticator::setOption('domain', 'bind_as','cn=SomeUser,ou=SomeOU,dc=fabrikam,dc=com');
ExternalAuthenticator::setOption('domain', 'bind_pw','SomePass');
domain is the short domain name