1. Is this a DVD that you have purchased and are trying to rip?
2. Is this an iMovie project that you have run through iDVD to burn a DVD?
3. Is it something else.

If 1., then you may be coming up against the encryption that is normal on a purchased DVD

If 2., then AND you have run it through iDVD, then you may have missed a step somewhere. To make a DVD that is playable in the average DVD player, the video has to be encoded first. (Usually by iDVD)

If 3., then we need more information