The PS/2 pins are D2 for DATA and D3 for Clock
PS2 mouse also needs 5V and ground

The pins used can be changed if required.

// PS2 mouse pins
#define DATA_PIN 2
#define CLOCK_PIN 3

#define I2C_PORT 41
#define SERIAL_BAUD 19200
#define HT_MODE 1

Assuming you choose I2C1 for the CMM2 end:
On my nano, the I2C pins are 
A4 SDA connect to pin 3 on the CMM2 and 
A5 SCL connect to pin 5 on the CMM2

For I2C2 on the CMM2 the pins are
A4 SDA connect to pin 27 on the CMM2 and 
A5 SCL connect to pin 28 on the CMM2

For the serial, I used a voltage divider to reduce the 5V on the nano to ~3.3V on the CMM2/micromite.
If you use COM1, connect the Tx pin via a divider to pin 10 on the CMM2.
For the voltage divider, I use 1.8k in series and 3.3k to ground or 1.2k/2.7k whichever I find first.

You don't need any connection to the CMM2 TX pin. We don't send anything to the Nano.
Serial is fixed at 19200 baud and is not required.

The default I2C port is 41 (decimal) but can be changed here if required.
The default serial port speed can be changed here if required
HT_MODE  1 = serial data in the same format as HobbyTronics interface.
         0 = serial data in a 4 byte compressed mode similar to PS/2 data
         
If signals are detected on the I2C, this is used and serial not sent.
If there is no I2C for 35mS, serial data is sent instead.



