Just download the "Aura Buttons" plugin from the "nUI: Unit Frame Panels" section at
http://files.nUIaddon.com and unzip the contents of that download file into your [
World of Warcraft > Interface > AddOns ] folder like you would any other addon. Also, make sure the addon is enabled (click the "AddOns" button at your character selection screen and make sure the "nUI_AuraButtons" addon has a check mark next to it).