You can do it one of three ways:
1 - in iMovie use the Share->Send to iDVD menu.
2 - use the Export menu option, select the movie parameters you want and save it to the Movies folder. Then drag the resulting file into the open iDVD menu.
3 - once the movie is completed, save and close iMovie. Locate the iMovie project file and open it by Control-clicking on the file. In the Contextual menu select Show Package Contents. Then navigate down to the Shared Movies/iDVD folder. In there will be a referenced movie file (media components are only referenced at this stage and transition, etc. have not been rendered) that you can drag into iDVD;s menu window. Doing it this way avoids having iMovie do the rendering of transitions, etc. before putting into iDVD. When iDVD does it's thing it will encode it it and provide a little better image quality than having it rendered/endoded twice.