How do i use "notebook' that now appears in "mailboxes'' in i pad 2?

Thank you

You don't. It's where the Notes app saves notes for syncing between devices. It was never meant for you to use with the computer itself.

While you can open it and copy the note text, you can't actually enter a note in that folder. Notes just won't see it, nor will the new note sync to other devices.

You're best off just ignoring it. Unless you want to grab some of the text while on, say, your computer. Whatever you do, don't delete them.