In the Robot->Motors and Sensors Setup dialog, click the Sensors tab. In Sensor Port S4, give it a name (e.g. HTMUX). Pick the sensor type "I2C Custom". Then click OK to update the pragmas. Then in the code right after the pragmas, put in the following lines:
Note that for sensors on the HT Sensor MUX, you need to use Xander's corresponding driver to access those sensors. You can't just use SensorValue[sensor] because Xander's drivers will initialize the sensors behind the MUX properly and it will translate the values to the corresponding sensor type if necessary.