Instead of trying to convert using your code, try mine located here:
Say your acceleration data is called "square_acceleration_data", then (once you save my iomega.m file in a location Matlab can find it, like your Matlab home directory) you would produce your displacements like this:
square_displacement_data = iomega(square_acceleration_data,dt,3,1);
where "dt" is equal to your time increment (i.e., 1/Fs or 1 over your sampling frequency).
Example:
