If you just put it in the drive and then the drive just does not accept it, its probably one of two things, bad DVD or bad Drive. Try another DVD, does the drive work? If so the DVDs must be bad.
You could try going into BIOS set it to start by DVD and not the hard drive, that may be the problem. Give that a try. To get to BIOS you hit an F key, repeatedly during boot, varies by company. Mine a Dell is f12.