Bear in mind that enrol_database_sync.php is not present in 1.5.x and that the associated code (in enrol/database/enrol.php) is missing too.
So you need either Moodle 1.6 or a backport of the1.6 enrolment code to 1.5.x. (not rocket science).
External Database Enrollment -- Is there a way to run the enrollment for all users?
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