I've never used MS SQL server, but I've read in the forums that the PHP setup is a bit tricky. Maybe you could have a look at http://docs.moodle.org/en/Installing_MSSQL_for_PHP for additional details. Even if the page talks about using MS SQL to run Moodle itself, the same setup (FreeTDS et al.) is required to connect to the external enrolment database.
Once you have it running and can make a connection from PHP to MS SQL, then you can try the enrolment part.