Xander's sample code for the sensors on the SMUX is pretty self explanatory. What kind of sensors are you putting on the SMUX? Each of the sensor drivers has specific functions to access the reading of the sensor. For example, if you plugged a gyro into the SMUX, then take a look at hitechnic-gyro-SMUX-test1.c. You will see the code calling the function:
That's how you access it. To enable a gyro to use SMUX, you need to include the SMUX driver. Then you need to define the SMUX port number.
Of course your include path may be different from the above. And also adjust your SMUX port number as appropriate.