24v Photocell Installation

I'm embarrassed to be asking this, because this thing seems to be simple as can be. I'm helping my sister replace a photocell that she had in an outdoor post light. I figured out the problem she was having as she tried using a 120v universal photocell switch. I went out and got a proper 24v unit. This is the first time I've ever used a 24v switch and I'm confused as to why it only has two wires vs three that the 120v uses. How should this be wired?