Instructions
Download Pitbull 4.0 from your Web browser from WoW.Curse.com. Press the "Addons" tab then type in "PitBull 4.0" under "Find addons." For ease of use, select "Desktop" as the download destination when prompted by your computer.
Open up the World of Warcraft program folder in your computer -- in Windows, typically at C:\Program Files\World of Warcraft; on Mac OS X at "Macintosh HD"/Applications/World of Warcraft.
Create a folder called "Interface" inside the World of Warcraft folder and then a folder called "Addons" inside that.
Drag the files that you downloaded from the computer desktop to the "Addons" folder you just created by clicking and dragging them from the desktop into the file explorer window you created. Once the file has finished copying, the add-on is fully installed and usable within your game.