You can't burn directly from iMovie, whose main purpose is to receive user's video and allow editing of it. Normally, iMovie output is passed to iDVD, then you edit DVD menus there. Don't know what version of iMovie you are using, but might be under Share menu to request passing to iDVD. If not sure, open iMovie help and enter iDVD as search term to determine correct way to pass to iDVD. When DVD menus are done, you can burn DVDs from iDVD. You want to use a good brand of DVD disk when burning (like Verbatim) and usually better to have slow burn speed. If have problems doing this, there is a support discussion forum at Apple web site. Go to Support->Communities, then go into iLife->iDVD section. Good luck.