All Empress pedals with the exception of the discontinued Superdelay use center negative power supplies. The Superdelay uses a center positive power supply. Please never attempt to power an Empress pedal with the reverse polarity as this could damage the pedal. Most power supplies will have a graphic like the following picture indicating what polarity the power supply is. You can find a list of compatible power supplies here. If you have any questions about if your power supply will work please contact firstname.lastname@example.org.
If you are using any Empress pedal besides the Superdelay the power supply should be center negative.