Apple no longer supply it.
Why is there no iDVD on my new Mac?
Apple insists that the entire world has access to fast broadband (and are prepared to pay for the considerable bandwidth usage) and wants to distribute home movies to friends and relatives via download (iCloud, YouTube, Facebook, whatever) rather than mailing them a DVD. The fact that in reality not all users do, has so far had no effect on this policy. If you scream and shout loudly enough down the phone Apple may send you a free copy of iDVD. Stating that they would return their brand new Mac unless they received a copy of iDVD worked for some, but that is now said to have been withdrawn by Apple. Also, you can complain bitterly here, perhaps suggesting that Apple could have provided a choice between burning DVDs and distributing home movies by other means:
Whilst Macs with a Superdrive continue to be able to burn video DVDs, the software for so doing, iDVD, is no longer included in the iLife bundle that comes with OS 10.7 Lion (which also omitted iWeb) or will come with OS 10.8 Mountain Lion. And it is no longer included in the iLife 11 from the online Apple Store: http://www.apple.com/ilife/. Your only solution is to look on Amazon or eBay and try to get an older version that includes iDVD 7, i.e. iLife version 9 onwards. You should also do this if you plan to buy a new Mac anytime soon, as stocks of iLife that include iDVD will not be available for ever.
However, the vastly more expensive FCPX can burn a DVD without iDVD or DVD Studio Pro involvement, but lack the themes etc of iDVD. Also, of course, there is Roxio Toast, which is the best software for burning anything but again does not offer the flexibility of iDVD.
And if you think Microsoft are any better, their new Windows 8 operating system will not play DVDs, or burn them, unless customers buy an extra upgrade, the company has announced: http://www.gizmag.com/windows-8-no-dvd-playback/22443/
In other words, computer manufacturers have declared optical media as dead, long before consumers are ready to stop using them, which is fine as long as they offered us a choice, but they won’t even do that. Flexibility and intuitive use of a computer seems to be a thing of the past.