When you installed Fog, it asked if you wanted to setup Fog as your DHCP server, and it configures it properly if you say YES. But you said NO, and it prompted you with instructions on how to update your existing DHCP server to modify options 66 and 67 (windows) or next-server and filename (linux). You must have missed these helper notes.
I think all you need to do is update your dhcpd.conf with the next-server and filename options for the scope definition. See: [url]http://www.fogproject.org/wiki/index.php/Modifying_existing_DHCP_server_to_work_with_FOG[/url]
Let me know if this helps. You may want to post your dhcpd.conf file (use code tags when posting to boards).