You said you're using an MPU9150? Assuming you're using the SensorHub booster pack then the TivaWare examples should show you how to configure the device to get all 9D of sensor output. The 9150 acts as an I2C master to the integrated magnetometer. When you read the 9150 you should get ~18 bytes worth of sensor data (16 bits * 3 axes * 3 sensors). Assuming, again, that you're using our sample code. This, however, should be hidden from you by the MPU9150 Get*Float() calls.
As for your constantly changing values, as CB1 pointed out, you haven't quantified your fluctuation. Did you enable any filtering on the 9150? What sensitivity are you running? 12 bits of sensitivity on 1G (9.81 m/s^2) gives you 0.0024 m/s^2 resolution (9.81 / 4096). In my experience at 2G sensitivity, the noise floor is around 0.2 m/s^2 or 0.02G's.
--Miles