On Macs with built-in Optical drives (with a few 2012 Macs as exceptions), a physical DVD must be readable and in the optical drive for that option to be enabled. You mention as 2013 Mac, as noted in my previous response, these models will support a direct installation using EFI boot and a Disk Utility-created Free Space partition. This Free Space will get split into an MSR and MS Data partition by the EFI installer.