The electric field can have visible effects only when it is applied to some material. So you need microscope slides with electrodes. If the electric field is transversal to the line of sight then pieces of metallic foil or wire could be used as electrodes. If the electric field is required to be parallel to the line of view then the electrodes could be two glass panes with thin (and therefore transparent) layers of a conducting material such as indium tin oxide used in LC displays. Of course, you have to take the electrical properties of the material under observation into account.

If you apply voltages above 60 V make sure that the microscope is connected to earth.