eject the boot cd, put in the one with the .bak file, then type:
mount /dev/cdrom /mnt/d
If you get no error message, it likely worked and this should show the .bak file:
The argument to -i on the restore command will be "/mnt/d/file.bak" .
edit: changed "backup" to restore in previous line. Also, if the mount fails, be sure /mnt/d already exists and create it if it doesn't:
If that says it does not exist, then:
and repeat the above.