Q: How should the electrodes be placed? In a symmetrical or unsymmetrical manner? A: The link you provide suggests the symmetrical pattern will have the desired function.
and my second question is: If these electrodes are placed symmetrically(as they are equidistant from the arms of the MZI) ( as seen in the Research paper enclosed), then how does the phase change between two optical signals occurs to get constructive and destructive interference? A: In Fig 2 of the paper, electric field is applied to upper and lower waveguides. The field of the upper waveguide is in the opposite direction from the lower waveguide. The EO effect is linear, so applying the external field in the opposite direction has the opposite effect. Therefore the optical phases at the second coupler become different, and destructive interference occurs.