For iPhone users:
I had trouble understanding above comments. I was able to add new comment like this:
-open new or existing note
-click "(i)" button (little round button top right)
-Click where it says "Add tag"
- > This is a text entry field, your keyboard will open
- > Do not use the "(+)" button, that's for adding existing tags
-Type the name of your new tag
- > Do not click the "(+)" button, or the "Close" link
- > On the keyboard CLICK RETURN to create the new tag
- > Then you can click "Close" link to go back to note
Done