I don't think this is possible because the Makerbuino uses the ATmega328 microcontroller, this doesn't support a direct link to the PC via USB. (The Makerbuino is basically an Arduino Uno without the interface chip).
I've not done this myself, But It is how ever possible using an Arduino Leonardo board, as this board can be setup to emulate a USB keyboard or mouse.
You could use the Makerbuino to send the commands to the Leonardo board, to which this would control your PC.
You could search on the Arduino.cc website for the information
This website which also explains how this can be done too.