Are you needinng to decompress RAR files on your mac?
If so what version OS are you running?
I can tell you how to use macrar, however I personally use and reccomend Stuffit as it has a much better level of integration an is a lot easier to use.
You can download a free version from here if you wish to take a look at it http://www.download.com/StuffIt-Standard-2009/3000-2250_4-10151590.html
If you would like the instructions for Mac rar let me know.