Project Notes have been removed from the Inspector to give you more control over how you create and organize your notes.
If desired, simply create a new folder called Notes (or whatever) and position it at the root level of your Binder (same level as Draft, Research, and Trash). You can add as many note documents to it as needed. For example, I like to have a list of basic character attributes, company names, and cities. Another note helps me keep track of the big picture timeline for the characters.
To make these notes easy to access, you can add them to the Project Bookmarks list, which makes them available from the toolbar or the Inspector. Simply open a document from the Project Bookmarks list in a Quick Reference panel or editor pane to view or edit.