Diagram shows four wires coming from my two wire power supply, what am I missing?
I think there are some wires coming from the photocell that get attached to the power supply, and some wires go from the power supply to the relay. It sounds like there are black, white, and red wires already connected to the photocell...the black goes to the + side of the 12V supply, the white goes to the - side of the 12V supply, and the red wire is the output. This is not a typical color scheme for low-voltage dc wiring so you should check the datasheet for the actual photocell you use.

In addition, a red wire coming from the + side of the power supply goes to the relay, as does a black wire from the - side of the power supply. It looks like the relay is wired so that the direction of the actuator will be reversed when the photocell output enables the relay. Again, you should make sure that the relay you actually have in hand is wired correctly...the locations of the terminals might not be exactly the same.