If it's a commercial DVD, then we can't help you as it's against forum rule #4 to discuss. However, if it's not, then download a program called Handbrake to rip the DVD into a format that can be imported into iTunes. If Handbrake is having trouble with the DVD, then use a program called MactheRipper to rip the entire DVD to your hard drive as a Video_TS folder, and then point Handbrake to that folder and it should work just fine.