Bitt,
I am by no means an expert, but the PC attached CD ones I've looked at tend to have two connections to the PC. One for data, scsi/usb/firewire - this connection is use to write to the disc by some a some what regular but customised CD burning program. The second connection is to control the robotic arm, which is controled by the custom CD burning software.

Now, you want to do the same but with your own software and with a DVD. The robotic systems do come with DVD drives, but that does not realy matter, as it is just going to show up as a drive in your system. The bit you need to focus on is the communication along the serial connection, load up a simple serial port sniffer on your PC and start playing with the robot from within the customised burning software - there are only so many movements the arm is going to need to make.
Once you know these commands, pair them up with some burning software that you can control and you'll be all set.