A rod  PQ is connected to the capacitor plates. The rod is placed in a magnetic field (B) directed downward perpendicular to the plane of the paper. If the rod is pulled out of magnetic  field with velocity $\overrightarrow{\mathrm{v}}$ as shown in figure.

Force on positive charge in the rod is given by $\mathrm{q}\overrightarrow{\mathrm{v}}\times \overrightarrow{\mathrm{B}}$.

Therefore positive charge will experience a force towards P and negative charge will experience a force towards Q. 
Therefore P is positive wrt to Q. Therefore plate M will get positively charged and plate N will get negatively charged.