It's easier for people to help you if you give plenty of information about your problem. Does it pop the DVD-R back out any time you insert it? Just when certain apps are open? Have you tried a different DVD-R?
Try burning your iDVD project to a disc image to eliminate some possible problems - that way you know it is just a problem with burning the disc. The use disc utility to try and burn the image. Drag the image file into the left-hand column and click burn. Disc Utility will ask for a blank disc. Try inserting then and let us know what happens.
Don't mean to be insulting, but are you sure your Mac has a DVD burner?