E-Health sensor shield:http://www.cooking-hacks.com/index.php/ehealth-sensor-shield-biometric-medical-arduino-raspberry-pi.html
Tutorial and documentation and schematic:http://www.cooking-hacks.com/index.php/documentation/tutorials/ehealth-biometric-sensor-platform-arduino-raspberry-pi-medical
Reading the documenation, it seems that sometimes the usb power is not enough. Could you try with a power supply for the Arduino Uno ?
The schematic does not show how it is connected to the Arduino. It seems that the new I2C pins are used, those are only on the Arduino Uno V3. Do you have that, or an older version of the Uno ?
And there are jumpers for the accelerometer. Did you set those ?
The MMA8452Q has an I2C interface, so the i2c_scanner should be able to detect it, once it is connected.