In the /tftpboot/fog/kernel folder resides the default kernel, it is called bzImage, this is the kernel the fog boot menu uses as I think that is hard coded in fog. If you cannot get your computers to register you will need to replace the current bzImage kernel, which according to Google is kernel version 3.2.4, with a kernel that allows you to complete the registration process. In order for your new kernel to work for the registration process it will have to be named bzImage.
So you will need to replace the default kernel with the kernel that works.
My suggestion is to use the GUI interface and find the kernel you need and download and install from there. Its the quickest and easiest way for me to do it.