This folder should be added to a hardware directory within your Arduino sketchbook. Normally the sketchbook will install to your documents/Arduino folder. If a hardware folder isn't already there, make it.
This directory contains a boards.txt file which defines how Arduino will handle ATmega128RFA1-based boards for the Quadcopter Class, and adds a selectable option under the Tools > Board menu. When you open the Arduino ID (close and reopen if it's already open), this option should be added to the menu.