Hi Daniella,
I can take a stab at this.
V = Ed, or 'Ed's Formula' describes the voltage across two parallel plate capacitors. It says that the Voltage (V) is equal to the product of the electric field between the two plates and the distance at which you'd like to solve for. Anytime you see a parallel plate capictor and are asked to find either the E, V or d, this formula would be the best to use given its simplicity.
As for units, with Electricity and Magnetism you should know that there are many equivalent units and that the MCAT expects that you know how to interchange between them. Voltage is typically measured in volts (V) but there are others: 1 V = 1 J/C = 1 Amp x Ohm.
Please let me know if you have follow up questions!