So what should you do first. I assume today you have the proper line in your current PXE boot menu to forward the client to the FOG server, Right??
Take a test computer and collect its mac address, then get with your networking or admin group and have them insert the line from your current pxe menu into a text file and title that file the mac address of your target computer. I think you create the file name without any punctuation. So the file name should be like: 0018DC234567 Then pxe boot the target computer and see what happens.
Actually read over this page it does a lot better explaining of syslinux pxe booting than I can: http://www.syslinux.org/wiki/index.php?title=PXELINUX Plus I had the search order backwards (go figure)