Part one: How to put Non-Commercial DVD into iMovie?
Step 1: Insert a DVD to your Mac. Open Disk Utilities. Find the name of your DVD in the left column list, select your DVD, and press "New Image" on the upper toolbar.
Step 2: On the screen that appears choose the location where you want your DVD saved (Desktop, for example), and choose "Read Only" under Image Format. Then, click Save.
Step 3: The conversion process should take several minutes. When your DVD is done converting an icon with your DVD name and ".dmg" after it will appear in the location where you saved the DVD.
Step 4: Open iMovie. Click on the ".dmg" icon. The process of Thumbnail Generation should start automatically and should take several minutes.
Step 5: Once the Thumbnail Generation is complete, a screen with various DVD tracks will appear asking you to choose which ones you want imported. Click Import All and wait for the Import to complete.
Part two: How to put Commercial DVD into iMovie?
If you want to put Commercial DVD to iMovie, you need to convert DVD to the video formats that are compatible with iMovie. I have tried Handbrake, which is totally free, but I pretty hard to figure out how it works. I currently use DVD Ripper for Mac , it is very easy to use and fast in ripping DVD to MP4. It supports converting encrypted DVD. Just follow these easy steps to convert and put DVD Movie into iMovie.
Step 1: Load DVD movies
Insert your DVD into your DVD drive. Drag & drop DVD disc icon on the desktop to the program. Or go to "File" menu, choose "Load DVD" to locate the DVD movies (support commercial DVDs and homemade DVDs) you want to add.

Step 2: Convert DVD Movie to iMovie on Mac OS X
Then select "iMovie" preset from output "Format" list. After setting the output settings, you can just click the "Start" button to start converting your DVD Movies for importing to iMovie.

Step 3: Import the output MP4 files into iMovie
Open iMovie and choose "File -> Import Movies" to import the converted Mp4 video. And then you can edit the movie according to your needs.
Free Trial Download Intel Version
Free Trial Download PowerPC Version
Related Tutorial: How to Convert DVD to iPhone on Mac?