Hi, I just bought a used imac 21.5" desktop. The model number is A1311. My problem is that it came with the HD deleted or the OS not installed, I'm not sure, and not very smart with computers and this is my first mac. It came with a grey Mac OS X Install DVD, but my drive keeps ejecting it about 30 sec. after I put it in. I am unsure how to get an OS installed on here. When I go to reinstall os x mountain lion it asked for a apple id that purchased the os, which I can't get from the previous owner. The only screen that comes up is one that says OS X Utility and has some options, but without this cd working, I'm not sure what to do. Thanks in advance for any help, much appreciated.