Can I add multiple notes in the same section?

No, you add only one note in a section. However, you can make that note as long as you'd like and also add bullet points for organization. You can also edit the note repeatedly to keep adding points later.

I added a note, but I don’t see it in the Lesson Plan.

The notes added in a particular step (Action, Analysis, or Application) will be available at the bottom of that step. Scroll down in the teacher's view to find your saved notes.