What folder is "FTP's MAIN directory"?
You need to place the files to a folder which is for your WWW files, eg. htdocs, public_html or whatever your server uses. Depending on your hosting provider/server settings FTP doesn't automatically connect to that folder but rather to your server user account home folder.
Do you see a phpBB page, 404 error or something else when you type in your install path on your web browser?