UnRarX is a great tool to have around. As the name would suggest, it will help you to expand rar archives.
I think that most Mac owners use Stuffit Expander for rar archives, but UnRarX has helped me when Stuffit Expander couldn't do the job.
Next time you you can't open up a rar archive, remember this post and come on back for UnRarX. (Or download it now and save yourself the trouble.)