Try Handbrake:
http://www.versiontracker.com/dyn/moreinfo/macosx/21117Ripping DVD's is one area where the PC has far more choices than Mac. Sad but true.
MacTheRipper is another alternative, but they "require" a €50 "gift" and even then, installing it and getting it to run reliably can be a nightmare. Read about it here before you jump in:
http://www.ripdifferent.com/forum/viewforum.php?f=26 Personally, I think MTR is a hypocritical ripoff. The sole purpose of the program is to get around copyright protection, but read their forums and see what they have to say about anyone trying to get around
THEIR copyright protection. They have the most involved, complicated licensing scheme I've ever seen, and unless you read through the entire forum about installation issues and problems, and try everything first, they won't help you.
I hate to say this, but as a Mac user if Handbrake doesn't work, your two best choices are to either download the movie you want instead of ripping it, or, as Mazuki said, run one of the PC rippers somehow.
There are k'd versions of MTR floating around, but if you find one you'd better read through their forums first because installing it is... ahhh...
interesting.