Use Teensy 3.1. Not Arduino. Teensy is far superior due to its higher flash size and far smaller physical size. You will need to incorporate some sort of optical sensor, the readings can be interpreted by the teensy, write a small sketch for this. Get the teensy adddon for arduino, get a teensy 3.1, set the teensy to appear as a mouse (can also appear as keyboard or joystick, or even all three at the same time) The teensy can then just be plugged into the USB port with the sketch loaded, the device should then start to function as a mouse.
I am in possession of teensy's, if you need any further help I am happy to answer questions. I am however not familiar with optical directional sensors, but can help with the teensy side.
I hope this helps,