I may simply be misunderstanding how this is supposed to work, but anyway…
I have a chapter up in the editor window, and I have the “Comments and Footnotes” sidebar showing. I want to add a comment to the text, and I’m assuming that the comment would end up in the sidebar. However, all shift+F4 does is highlight text in the editor. The “+” in the sidebar does nothing. I tried with footnotes as well, and nada. +fn does nothing as well.
First, make sure you’re using the latest version of Scrivener (1.8) by running Help > Check for Updates. There was a bug with the inspector focus a few versions back which could cause what you’re seeing when working in Scrivenings mode, I believe. You say you have a chapter loaded–is this in a Scrivenings session, with multiple documents in the editor (e.g. you selected the folder, so all its subdocuments are open in the editor)? Or do you have a split editor? One possibility in the latter case is that the inspector is locked to the other editor. Check in the right of the inspector footer to make sure the padlock icon isn’t set, and try switching to the document notes and synopsis view to check that the correct document’s title appears there.
Argh! I’ve just figured out the issue. Comments work fine when I’m working with a file, but when I’m working with a folder (with text), the comments don’t work. I can convert my folder to a file, insert some comments, then convert it back to a folder, and it seems to keep the comments (so far. I haven’t tested this exhaustively yet) so I have a workaround.
OTOH, I see that the new release fixes the <$rst> bug, so I may not need to keep some of my chapters as folders anyway.
Comments and footnotes should work on a folder’s text the same as regular document’s, so there’s definitely still something off with what you’re seeing. When you have the folder’s text loaded, is the left group-view mode button highlighted in the toolbar? Like so:
If so, try clicking that button to switch it off and see if that makes a difference when creating your comments and footnotes. It should work either way, but this will help narrow down the issue. A screenshot of the whole project window at the point where you click the “+” button to create the comment might show something helpful, too, if you’d like to share that.
OK, I can replicate this on demand. Here’s the info: Win7, Scrivener 1.7.3.0
I select a chapter that is formatted as a folder with text, in the binder. Put the cursor in the editor (only one editor open), and the headers of the Comments and Footnotes sidebar turns black. However, if I click on the “+”, nothing happens.
If, however, I click back on the folder in the binder, then click back into the editor, the “+” will work. It looks like the first time you select a folder in the binder, the comments won’t work. But if you click out of the editor then back in, it works. Until you click a different folder in the binder.
Okay, I can get what you’re seeing but only when the folder loads in Scrivenings mode. So until we’re able to fix this bug, switching to load the folders in single text mode instead of Scrivenings should avoid the problem. Once you’ve toggled off Scivenings for a folder, that will be the new default for loading folders; i.e. you’ll only need to do this once, and your other chapter folders should then all work when you first load them.
Or just click in and out as you’re doing, whatever you find easiest.
I just downloaded 1.8 , and it doesn’t appear to have the problem, at least not the way I’ve been trying it. That may be a source of the apparent confusion.
You can add that to the list of bug fixes for 1.8, maybe under the heading “Inadvertent Bug Fixes” 
I sometimes have this problem (July 2020, Scrivener for Windows v 1.9.16.0). The + and - signs for Comments & Footnotes are greyed-out unpredictably. The workaround is to convert a folder to a file or vice versa. Notes now work. If you then convert back to the original document type (folder or file) notes still work.