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.