Well, put in your DVD and see if iMovie will be able to read them (try to import one). There are a few different formats and iMovie can't read some of them; in that case, try MPEG Streamclip (free, but you need to read their requirement for the MPEG2 Component on their website) to convert the footage to something iMovie can address.
Long story short: video on DVDs can't be edited/manipulated. It's compressed and encoded in a manner that they are basically proprietary for DVD use only.
So you have to uncompress/convert them into something that can manipulated.
You can use either a program like DVDtoiMovie
or a freebee like MPEG Streamclip.
Both give you options to convert DVDs into an editable format. For IMovie, you'll be wanting export it into DV (which will uncompress your footage into very large files.
You need Apple's MPEG2 component which may be purchased at the Apple Store Online (www.apple.com/store). Note that you do NOT need "QuickTime Pro" as this is an entirely different animal.
Once you've installed the MPEG2 component, any VOBs may be accessed directly though any QuickTime-enabled applications. If you have a DVD that is not copy-protected, drop the first ".VOB" that contains the first part of the footage onto MPEG Streamclip (free); when it asks you if you want to open the whole movie (or something like that), do it. That will bring the entire title (all the footage in all the VOBs) into the MPEG Streamclip window where you may cut out what you don't want and then "Export to DV...". That will create the DV-Stream footage iMovie likes. Drop that into iMovie and go for it.