Don’t forget to exportfs -a, I know I’m being captain obvious here, but I just want to make sure jbonilla doesn’t skip any steps.
Just for comparison, this is how I boot:
:ubuntu
kernel http://${fog-ip}/fog/iso/ubuntu64/desktop/casper/vmlinuz.efi
initrd http://${fog-ip}/fog/iso/ubuntu64/desktop/casper/initrd.lz
imgargs vmlinuz.efi root=/dev/nfs boot=casper netboot=nfs nfsroot=${fog-ip}:/var/www/fog/iso/ubuntu64/desktop/ locale=en_US.UTF-8 keyboard-configuration/layoutcode=us mirror/country=US
boot || goto MENU
with chmod set to 755 on the folders. Btw, your vmlinuz and initrd should usually be located in the casper folder.