You could use the http://www.creationkit.com/OnStoryIncreaseLevel_-_Quest event for that. Create a quest that adds the spell to the world or your player in its startup stage, make IncreaseLevel its start event, put it into an Story Manager event node with the condition that the level is 20.
And I would strongly advise you
not to put all your questions in the same thread, but to put different topics in separate threads which have titles saying something about what the problem is. That lets people know whether the thread is worth them reading, either because it's on a topic they know about, or because you have a new problem and are not just going on about the old one they've read about before and, for whatever reason, not replied to.