You may need to build a "DC condom". Input power plug that fits the AC adapter plug you are using, a full wave rectifier (or 4 diodes), a decent sized electrolytic capacitor to convert AC to DC and filter out ripple, then the output plug that is 5.5mm and 2.1mm that fits standard Arduino power plug. The condom will also work if you have a DC adapter with correct sized plug but has reversed polarity.