The DVD footage will be in Mp2 format, and you will need to convert the underlying VOB (video object) files into
something that iMovie can read -- like Mpeg-4, or DV, or .mov.
So, insert your DVD into your computer's DVD reader. When the DVD icon appears on your desktop, click on it. If it automatically plays the DVD, close it. You should see a folder that contains two folders that look like this:

Your movie files are located in the VIDEO_TS file. (The Audio_TS file is empty and you can ignore it.)
Click on the VIDEO_TS file. You will see something like this:

The files with the .VOB extensions are your video files. Option-drag them to your desktop, which will copy the files, not move them. You want to leave everything in you DVD folder intact. Then take your copied VOB files and convert them
to one of the codecs I mentioned above, using a free download program like Handbrake or Streamclip. The converted files will play in iMovie and can be edited.
This process should be used only for your own personally generated vids and DVD's, and not for commercially made vids that may have copyright protections.