You didn't mention which Arduino you have, but it should be simple on the Leonardo, since it drops having the secondary processor for USB communication, and can appear to the computer as a USB keyboard, USB mouse, or serial port. http://arduino.cc/en/Reference/MouseKeyboard.
The Due has a secondary USB port that can be used as a keyboard.
For the Uno and Mega, I believe you can use a USB host shield to emulate a keyboard. Here is one such USB shield: https://www.sparkfun.com/products/9947?. Obviously, if a shield exists, you could replicate the important bits as stand alone components, but the shield can be more convenient than doing it yourself.