Simulating accelerometer noise
The sensor specifies its noise in terms of the input signal (acceleration), so the first thing you need to do is convert that to equivalent voltage noise, taking into account the gain settings you're using in the sensor and the interface circuitry.

The signal power in a series of normally-distributed samples is equal to the variance, which is the square of the standard deviation (sigma, &sigma;). However, the numbers in your simulation represent voltages (or currents), not power. Therefore, you want to set &sigma; equal to the equivalent noise voltage you calculated above, and then add these numbers to your simulated acceleration signal.