Raspberrypi Wireless Attack Toolkit uses Raspian as it's base OS. You can download that here
To install the software, you can pull down on the Git repo and follow the instructions below or you can download the network installer file from the downloads page of the sourceforge project.
1.) Download the .tar file or pull from git:
git clone git://git.code.sf.net/p/piwat/code piwat-code
2.) Go into the install directory inside piwat-code
3.) Run the installer script
4.) Watch in amazement as precompiled libs are cp'd, packages are installed, and stuff happens. When stuff stops, you're ready to go to piwat-code and run the start.sh script to configure and start piwat.