This is a library for use with the Si4463 radio IC from Silicon Labs. The Si4463 is used in a number of pre-made modules like the HopeRF RFM26W and Dorji_Com DRF4463F. The whole range of Si446x transceivers should also work with this library.
http://blog.zakkemble.co.uk/si4463-radio-library-avr-arduino/
Copy the arduino/Si446x/ folder to your Arduino libraries folder.
Add
#include <Si446x.h>
to the top of the sketches that use the library.
Also have a look at Si446x_config.h, that's where you can change which pins are used and stuff.
Copy the ./Si446x folder to your project and add
#include "Si446x/Si446x.h"
to the source files that use the library.
Check out the examples in the examples folder.
Zak Kemble