Unless between 502 and 503 the Grub4DOS system has been changed, the following should help you (as its the code I use in 502):
title CloneZilla\n Load CloneZilla to backup or restore your hard drive.
kernel /ubcd/custom/CloneZilla/live/vmlinuz live-media-path=/ubcd/custom/CloneZilla/live bootfrom=/dev/sr0 boot=live union=aufs noprompt ocs_live_run="ocs-live-general" ocs_live_extra_param="" ocs_live_keymap="" ocs_live_batch="no" ocs_lang="" vga=792 ip=frommedia
However, as you can see, I have had to extract the ISO as I was having similar problems to you.
If you can deal with a bit more hassle you might be able to streamline the kernel parameters, but the current compilation works fine.
In case you're new to this, I should probably make it clear that you need to add the above code to /ubcd/custom/custom.lst
and use the Grub4DOS menu system to access it when testing/using UBCD for it to work.