I found these two lines of code in the setup( ) which it probably needs. The "mpu" here is a class object for the MPU6050 device which is based on Rowberg's with some modifications. I also use a different I2C implementation but I don't think that is relevant here.
mpu.setMasterClockSpeed( (uint8_t) 13 ); // set auxilliary I2C clock rate to 400 kHz.
mpu.setI2CMasterModeEnabled( true ); // turns on the secondary I2C