the issue you are having does not have to do with the medium your booting with. you passed POST, you are now at bootloader (grub), thus system is booting and needs to be told where to boot to. grub has most likely (am not positive) attempted to boot but cannot and needs to be told where to boot to. check DVD burn? test USB-external-DVDROM drive + current DVD burn with different machine? just my 2cents.
otherwise try to manually boot. this is what grub on the live-DVD is scripted to do...
note: this is using current R2 build
grub> kernel /boot/vmlinuz BOOT=casper boot=casper nonpersistent rw quiet
grub> initrd /boot/initrd.gz