How do you turn on the inspector window

the F8 key turns on the Inspector (fn F8 on Mac)

The Inspector window should always be present unless you close it. If you've closed it, you can bring it back with View / Inspector. Or the shortcut F8. Either of these is also a way to close it.

It's possible what you are really seeing is that the Inspector is there, but empty. The Inspector contents change according to your selection. With nothing selected, it is empty except the words "Nothing selected". With a single element - or multiple elements of the same type selected - you see the usual controls relevant to that element type. If your selection includes elements of different types, then all you'll see if a few generic controls like color and visibility. If you've selected a range and you really just want the notes within that range, you should see a "Notes" button at the bottom that will limit the selection to just the notes, then you'll see the regular note controls.