Quick Note is a way to add new entries or append to existing entries quickly from any app. It is invoked with a key combination that you set up. Once invoked, a window appears floating over whatever app you in and letting you quickly add something so you don't have to leave the app. To set up Quick Note, first open the General preference pane. The Quick Note setting is at the top. Click the "Change" button and then type any key combination your keyboard that you want to use for Quick Note. MacJournal will remember that key combination and use it for Quick Note in the future. If you want to change it, use the "Change" button again or the "Reset" button to clear it and disable Quick Note.
When you invoke Quick Note, a window shows up with a few areas that you can use to add information to MacJournal. At the top, you can pick whether you want to append the text you enter to an existing entry (with a pop-up that lets you pick which entry) or create a new entry with (with a pop-up that lets you pick when journal to which to add the new entry). Below that, you can set the topic and tags of the entry. When appending the text to an existing entry, the existing topic and tags show up and can be edited. Below the metadata fields is a text area. Any text you enter here will either be appended to the selected entry or created as a new entry.
When you are done, you can use the Cancel button to discard any changes or the OK button to accept them. Pressing the Quick Note key combination again will act like "Cancel". Pressing Command-Return will act like "OK".