When you go into "Sensors" in Matlab Mobile, you can see variables measured by your mobile sensors in real-time. You can simply move your mobile phone in some of those directions: front of the car is the x-axis and y-axis is the pointing to right door and z is down. Moving your phone in any of the directions will detect the acceleration and reveals a relevant axis. Then, when you are familiar with your phones axis, you can adjust mobile phone in your car to match your requirements.
Most likely though, the axis on your phone will be as in this picture:
Source: Sensor Data Collection with MATLAB Mobile - MATLAB & Simulink (mathworks.com), that might be a useful link to check.
