I ftp my website, which contain files within sub folders to the "/public_html/" folder. When I tried to access it via http://jw.elementfx.com/lightbox/index.html, it gave me a 404 not found error. Can you please explain to me why? Thanks.
You have created your hosting account very recently and the server(webserver daemon) hasn't refreshed itself yet to reflect this.
Hence,you are getting this page.
For more information, please read this x10 wiki: http://x10hosting.com/wiki/Defaultwebpage.cgi