Unless you are chaining together that login to SilverStripe with another external source, it's pretty pointless using external authentication just to check SilverStripe users - as there already exists a perfectly adequate login for SilverStripe. The point of the SilverStripe driver for auth_external is to allow daisy chaining logins with other drivers - and wasn't meant to be used for individual login. Either way, if you enable the SilverStripe driver, you should be OK. You can achieve this by copying the example information from the docs/sstripe.php to _config.php and changing the details inside to match your installation.