It actually is easy to get to the optical drive. You should disconnect the power cord from the PSU and disconnect your monitors, and any other peripherals. Lay the computer down on its back side and open the PSU swing-out contraption to expose the innards. Follow your red data cable to the back of the optical drive. I cannot tell if the data cable is connected to the optical drive because the PSU is in the way, and the photo is small. The 6-Pin Slimline SATA power adapter connects to the optical drive immediately adjacent to the SATA data cable connection. You have 2 drives in the bottom of your case. Each of those drives has a SATA power connection direct from the PSU. One drive has a blue SATA data cable, and the other drive has a black SATA data cable. Your PSU should have come with 4 SATA power cables that connect to your PSU in the ports labeled "Peripheral & SATA", and each of those cables should have multiple SATA power connectors. Since your 'visible' SATA power cables are both routed down to the bottom of your case to power those drives, you will need to route another SATA power cable to the top portion of your case to connect the 6-Pin Slimline adapter to SATA power, and the other end of the adapter to your optical drive. Always triple check that your cables are routed such that no cables are being pinched by the PSU swing-out contraption as it is opened and closed. If you have never swung out your PSU swing-out contraption, this is what it looks like. (click photo to embiggen) [IMAGE]
I had to have a new PSU installed. They told me they could not hook up the optical drive because there was no wiring for it that came with the Corsair CX750M. Told me I would have to find an adapter online or just use an external usb drive. I really didn't want to do the external drive. Can you tell me what I need to make this Corsair work or should I get a different PSU that would have the correct hookup? I really don't even know what the optical drive in the XPS 8930 is. 
XPS 8930, new Corsair PSU CX750M, no power to optical drive