Here are two pictures that show how to wire a mouse with PS/2 connector and a PS/2 compatible USB mouse to the Arduino Nano or UNO.

The data goes into the Arduino's Rx pin, clock goes into pin 4. With the 10k resistor between Data and +5V, it is still possible to use the interface for uploading programs while the mouse is connected. After development is done, the resistor can be removed.

There is a small example sketch that lets you control some LEDs with a mouse. Wire the example like shown below:

Then, all that's left to do is download the example sketch from Github, and flash it on the Arduino.