Remove the side of the computer, remove the cd tray, loosen the screws that hold the cd player into the tray just a little bit.  Reinstall it and try.

Sometimes the screws are too tight from the factory and it warps the drive just enough that it doesnt work.

Yep, having the same problem here. Yet to hear from tech support email, so I'm going to try this tonight.

This issue was being reported back in February. Can someone tell the robots in Poland not to screw down the ODD screws so hard?

I'm going to leave you an article with possible solutions for optical drive issues in the X51. It includes the one Kameix kindly explained and other useful steps.

Alienware X51 – Solutions to Optical Disc Drive (ODD) Issues

Take off the side cover and make sure it is plugged in.,

haha it is I'm sure they had the sense hook everything correctly, makes me think that the optic cd drive out allighn were the sensor is not responding to accept the CDs

Below is some of what I sent to TechSupport. And my chances of getting a replacement optical drive are?

"After looking on the internet this weekend I found that many users of X51 builds were experiencing simillar problems since february. I decided to open the Chassis and examine the physical installation of the optical drive. It soon became apparent that the drive had been damaged during installation with an obvious narrowing of the apature, a 'pinched' effect as if the unit was squeezed at the top and bottom. I gently increased this constriction to a more rectangular shape and reinstalled the drive.

On assembly and power up I discovered the dvd drive is now functional. I have watched a dvd and installed a DVD game.

However the unit seems to be overly noisy. Could you advise your assembly line operatives to take extra care. Modern hardware seems to be becoming quite flimsy."

I called technical support and talked about the issues I was having, they didn't hesitate and went ahead and sent optical drive replacement for me. It seems calling technical support and addressing your current issue to them will present you a option for a cd drive replacement altogether. I do believe after read the post above that this a damaged drive which was assembled in there factory line, there probably having more people confront them with issue by calling tech support.

Besides calling tech support, you can always send an email to awsocialmedia@dell.com with a brief description of your issue, include the service tag as well and our team will review the case. You may not have to wait on the phone that way :D

Update: Sorry it's been a while but I've been enjoying my new X51. Tech support europe shipped a new drive promptly after I sent a picture of the damaged one (after returning it to it's 'damaged' state). Installed smoothly and running fine.