Hi,
It's hard to tell like this, but yes, you should get the acceleration value and the next action must be a float multiply that multiply by -1.
if that's not the case, then it might simply be a problem of rig set up and transform.
I suggest the following. take control of the value yourself without the accelerometer, and see how the cube is behaving. if it behaves find, either you are mis interpreting the accelerometer value or something is odd in the process. you can do that with a joystick for example, or simply with the keyboard.
Also, do you have some debug tools to watch the values of the accelerometer, so that you are actually sure about the outputs, this does help a lot as well.
Bye,
Jean