Are you running Windows?
Is it the version that was shipped with the system? Or did you do anything like nLite the OS before you plugged the DVD in?
With the DVD plugged in, right click on My Computer and Select Properties. Click the Hardware tab, then the Device Manager button. Look thru the list and see if there are any exclamation points. Look for the DVD drive. Is it there? When you double click on it what does it say? Could be you need to install a driver.
One thing you might do if it shows up in the device list, but isn't recognized for some reason is right click it and select Uninstall. Then reboot the machine. Now does it work?
Still no? Hook it up to the internet, then right click on the device, find the device driver tab and click update driver or whatever. Tell it to feel free and look on the internet for the driver.
Now does it work?