My understanding (and hopefully more experienced Moodlers will correct me if I'm wrong) is that Moodle 2 does not automatically email students who have been manually enrolled. While this is inconvenient at times, you can workaround it by using the Participants list.
Within your course, go to Navigation block -> Course short name -> Participants. From here, you can select all those students in your course who have not yet logged into your course, and send them a message, letting them know they've been enrolled. It works best when your site administrator has forced users to receive email notifications of Personal Messages between users, to make sure the student receives the message in their email inbox (in case they have not yet been on the Moodle site).
For more on this, you can see the docs link here:
Or for a walk through, you can watch the video here:
I hope this helps!