You may have made the directory in the wrong place. In Debian at least, for your main site you would want to mkdir /var/www/newdir
or for individuals mkdir /home/user/public_html/newdir
To access the first one you would put www.mysite.com/newdir
in Firefox. For the 2nd one, it would be www.mysite.com/~user/newdir
Also, this may be a DNS problem where the browser doesn't know what ip address is associated with www.mysite.com
. Can you access your mainpage with the browser?