In iMovie 10 go to File>Share>File, choose the size (probably should choose either SD or Large) then click Next, give it a name, then click Save (remember where you saved it to).
Wait until the Activity Indicator shows that the sharing process is complete.
Go to iDVD. Go to File>Import>Video and select the file you created earlier. It will bring the video into your iDVD project. Prepare you DVD project and when you're ready, click the "Burn" button.
iDVD is no longer officially supported by Apple, but that doesn't mean you can't still use it. It's still a great option...possibly the best...for burning video DVDs on a Mac.