If you are still within your 15 day return period. Call Apple they will sent out iDVD at no charge. (There are no realistic substitutes.)
Nicely explain that you may return the computer without iDVD. That usually does the trick. You MUST speak to a senior advisor to get a free copy. The first tier people cannot do it.
If that doesn't work you can purchase iLife 11 (or iLife 09) (includes iDVD) on disk.
IDVD is a wonderful piece of software and well worth the low cost of $40.