Directories:

+ FreeRTOS/Source contains the FreeRTOS Multicore real time kernel source code.

+ FreeRTOS/Demo contains a pre-configured demo project.

+ See http://www.freertos.org/a00017.html for full details of the directory 
  structure and information on locating the files you require.

The easiest way to use FreeRTOS is to start with one of the pre-configured demo 
application projects (found in the FreeRTOS/Demo directory).  That way you will
have the correct FreeRTOS source files included, and the correct include paths
configured.  Once a demo application is building and executing you can remove
the demo application file, and start to add in your own application source
files.

See also -
http://www.freertos.org/FreeRTOS-quick-start-guide.html
http://www.freertos.org/FAQHelp.html