Mroogle will have answered that question for you many and plenty of times.
There is MacTheRipper, RipIt and Fairmount to rip (copy the DVD to your HDD while removing the copyright protection) the video DVD to your HDD.
Then there is Handbrake to convert the ripped DVD to a file like .mkv, .mp4 and .avi with MPEG-4 codecs like Xvid and H264, which are not meant for editing though, as they don't store every frame of the video (video DVDs use MPEG-2 as a codec, which also only stores every 15th frame and the frames in between are approximations).
After that you can use MPEG-Streamclip to convert the compressed video file to a .mov file encoded with the DV codec, a codec iMovie can read and is meant for editing, as it stores every frame and takes up approx. 220MB/s.
You can also skip Handbrake and use MPEG-Streamclip for converting directly to a DV encoded .mov file from the ripped video DVD, but you need the QuickTime MPEG-2 component (19USD) to be able to access the MPEG-2 encoded video DVD footage via MPEG-Streamclip.
But it would save one encoding process.