Which step is this again? There is no step referred to as “5 - 3)”—and the closest thing to that, “Step 5c: Notes” concerning Document Notes in the Inspector—doesn’t match what you are talking about in terms of steps to be taken.
I see everything that you see, but I am unable to enter any text, even when I click on the “+” button at the left hand foot of the page. When I click on “OK” the drop down disappears.
If it isn’t working at all – and if you have tried the standard troubleshooting steps of (1) closing and relaunching the program, and (2) rebooting your Mac – I would try creating a new test project (using one of the standard templates) to see if the + button works as it should in that project. If it still doesn’t work, I would uninstall Scrivener, reboot the Mac, grab a copy of the latest Scrivener build, reinstall the software, and then try again.