I'm struggling with that myself, since my currently favorite save isn't likely to support magic.
You could go the good old "it's technology, really" route. Did you watch the lifestream where they showed the two-way portals? They could have been created by technology or left behind by aliens, no need for magic. And the whole magical realm might be a magic themed funpark that your sims visit with their kids. Clever illusions, fancy sfx effects...
Then, once you've seen how everything works, you can slowly introduce real magic into your gameplay.
