Where is iDVD on Lion?
iDVD is only available through physycal media copies of the latest iLife package.  Preloaded iLife suites no longer include it, and I do not believe you can download it, either from Apple or as a paid product via the Mac App Store.  But if you have the media from an old machine, you should be able to install it happily enough though, it's not changed since iLife '08, so almost any media will do...

Bear in mind that if you are using for example the bundled media from an old Mac on a different Mac, chances are that the installer package is tied to the original Mac. But it's trivial to work around. Right click on the installer, click Show Contents. It will show you the contents of the installer program, which is just a wrapper around the actual packages. Somewhere there is an ilife.pkg for installing everything, but also individual ones for each app including iDVD - I don't have anything to hand to check the exact names or locations, but hunt around. Then run the individual packages for iDVD (same goes for iWeb which is also deprecated in the same way).