This video tutorial will show you how to make a simple circuit and download a library that will make any ATMega328 based Arduino into an USB HID Keyboard. We show of the simple demo sketch that comes along, that should give you an idea of how to use the library. IMPORTANT: You need IDE version 1.0.5r2 or EARLIER to make this work.
You will find the schematics here:
www.practicalarduino.com/projects/virtual-usb-keyboard
You will find the library here:
https://code.google.com/p/vusb-for-arduino/