I have edited the functions file successfully but now I don't know where to go.
I thought I'd see a custom field appear below the area for the post content marked custom field but I don't see one.
According to this article on WordPress, it's hidden by default until used for the first time but I can't find where to create it. http://codex.wordpress.org/Custom_Fields
I'm doing this so I can enter a dynamic javascipt banner in the post area.