Meal Notes will show up if your client views a meal plan on Eat This Much (both web and mobile apps), and will also show up if you create a PDF or email out of a meal plan. If you copy a meal plan, the notes will be copied with it.
Here's how to add notes:
You can click the note or use the same button to make edits.
When you create a note, it will look like this in the plan:
You can also switch to your client's accounts to leave notes directly in their calendar of meal plans. Even though you are using their account, only you can leave notes as the Pro user (and the client cannot create meal notes).
Meal Notes will persist when regenerating a meal (and even when regenerating the day or week). If you'd like to remove a note, simply click it, and then hit the "Delete" button in the editor popup.