There is no direct encoding out of FCP. Quicktime/Compressor does that job. It takes the .mov file to get out an MPEG-2/.aiff file that will go into a DVD authoring application.
While both iDVD and DVD Studio Pro allow you to just drag that .mov file into the app. it is doing the encode in the background. If you import an unencoded file into DVDSP, you will get a red indicator in the browser next to that file name indicating that it is being encoded.
I find it best to export a self-contained movie file including the Chapter Marker pulldown.
Open Compressor and drag that movie file into the Batch window. You should see those chapter markers indicated at the bottom of the viewer. Click start to encode the content. Then open DVD SP and import those video and audio files into the browser.