When creating a case you have to make sure your object type is an impression, then you must select a tooth (even when you're not working on one). Then you'll be able to select Model on the right side of the screen. From here you will import your scan data. This may not be exactly what you're looking for but this was how I was shown. By the way don't feel like an idiot, we've all got learning to do
P.S. Always make sure your case has the little yellow exclamation point on the scanner icon. That's how you know you can use model builder with it.