To install rdiff-backup you need to execute the following steps:

1. Install Apple's Xcode programing suite, available as
   a separate installation from your OS X CDs or DVD,
   or preferably the latest version from Apple's Developer site:
   http://developer.apple.com/tools/xcode/index.html.
2. Install MacPorts from http://www.macports.org.
3. Open `Applications → Utilities → Terminal` and type in the following
   commands (you may copy them line by line from here,
   paste them into the Terminal, confirm the command
   with `Enter` and wait for every command to finish):
   `sudo port selfupdate
   sudo port install rdiff-backup
   sudo bash
   echo "setenv PATH $PATH" >> /etc/launchd.conf`
4. Restart your system.