how to open rar files?

there are many unrar programms.
e.g. http://www.unrarx.com/

http://en.wikipedia.org/wiki/RAR

Third party programs that can only read (unpack) RAR files include: WinZip (Windows), RarZilla (Windows), 7-Zip (multiplatform), IZArc (Windows), PeaZip (Windows, Linux), Zipeg, (Windows, Mac OS X), ALZip (Windows), along with the free version of unrar (which is available for Linux and FreeBSD). Mac OS X readers include Stuffit Expander, The Unarchiver and Zipeg. Stuffit Expander is also available for Mac OS Classic with RAR support for this platform.

Zipeg, unrarX and RAR Expander are the most common OS X (unpack only) programs.
http://rarexpander.sourceforge.net/
http://www.unrarx.com/
http://www.zipeg.com/mac.video.html

If you want the rar compression utility,
it is available in command line form
from the developer with a trial license:
http://www.rarlab.com/download.htm

The Archive Utility.app inside /System/Library/CoreServices/ should do the job. If not, then the free StuffitExpander at http://www.stuffit.com/mac-expander.html will.

Then, use the free expander.