In a terminal type "dmesg | grep dvd" The dmesg command prints out your bootup messages, piping it through grep helps to find what we are looking for (in this case, the dvd drive).
On my computer, the output looks like this:
[ 1.865754] sr0: scsi3-mmc drive: 40x/40x writer dvd-ram cd/rw xa/form2 cdda tray
To create the link I would then enter the following:
sudo ln -s /dev/sr0 /dev/dvd
(substitute sr0 for the device name you found in the previous step.)