Dear Sir or Madame,
It was just announced a few days ago that internships were closed. No more intenships [sic]. Meaning, as I understand it (and I've checked as I'm in the same boat as you), that if you haven't added the course they aren't going to let you.
But I could be wrong.
And this, ladies and gentleman, is why I panicked about my for-credit internship a week ago, when they told us that they were closing all registrations for people who hadn't registered for class(es). Since I panicked immediately after the email went out, I managed to get my adviser on the line and sneak into the for-credit internships under the wire.
This is also why you should make sure you're on the official announcement list for whatever programs you're in. You actually find out if they're planning to fuck you over and when.