The DVD Won't Mount or Eject on a Mac
If you've inserted a DVD into your Mac but it's not being recognized, you need to try to force-eject it. Mac OS provides a few keyboard shortcuts that can help eject a stubborn CD, or at least let you open a utility that can eject the CD from there. If those options don't work, you can reset your Mac's PRAM or NVRAM -- depending on your Mac model -- and the System Management Controller to force your computer to recognize your DVD.
1 Basic Troubleshooting
There are multiple different ways to try to force-eject a CD or DVD from the drive that don't require changing settings or resetting any data. If the DVD is mounted but will not eject, click and drag the DVD's icon to the Trash. If this doesn't work or the DVD isn't mounted at all, restart your computer while holding down your mouse button or trackpad button until the DVD is ejected. You can also hold down the "Option" key as you restart until you see the Startup Manager, then select the DVD icon and eject it from there.
2 Reset PRAM/NVRAM and SMC
If none of these options work, you can attempt to reset your PRAM or NVRAM and SMC, then try to mount or eject the CD again. To reset your PRAM/NVRAM, restart your computer while holding "Command," "Option," "P" and "R" simultaneously. Hold them down until you hear the startup chime twice, then release the keys. To reset your SMC, shut down your computer, unplug all cables and remove the battery. Press and hold the Power button for 30 seconds, then replace the battery, plug in your cables and start your computer again. If the DVD still refuses to eject, you may need to visit an Apple Store for technical support.