Opening .rar extension files are not available by default in Mac OS X. You can use the free tool named “Rar Expander” to unrar the rar files. RAR Expander is a MacOSX program which extracts the files contained in RAR archives. It supports both single and multi-part archives, and has support for password-protected archives as well.
It uses the official unRAR library internally so it is fully compatible with archives produced by WinRAR.
RAR Expander also features AppleScript support, and includes a few useful example scripts for expanding multiple archives at once.
After installing RARExpander.dmg file in your Mac OS X, just double click on the rarexpander and select File -> Open to locate the .rar file that you want to unrar. Select the destination where the rar file to be extracted.
RAR Expander Download
Note: MacOSX 10.3.9 or above is required for running the latest version of RAR Expander. If you have an older version of MacOSX, you can find earlier versions of RAR Expander on the sourceforge file list page for RAR Expander. Version 0.7 should run on OSX 10.2.