Is this for a planted tank? I have a RKL, just upgraded to a RKE for a new build I'm doing.
My RKL was on a planted tank to control pH/CO2. So you have the reefkeeper lite, one of the power strips and a module that can measure pH with the probe? I believe both SL1 or 2 can read pH and will work for this.
Solenoid for the CO2 regulator gets plugged into one of the outlets. You then need to set up that specific switch. If your pH is normally 7.5 most people want a 1 point drop for planted tanks, so you'd aim for 6.5. So you set it up saying, if pH is above 6.5 the outlet is powered which opens the solenoid and flows CO2. Below 6.5 it cuts power which closes the solenoid.
You'll need to test your range a bit, tanks can all take different amounts of CO2 before the fish start gasping, but that is the general idea. I found setting it up using a computer to be much easier than trying to use the interface. Hopefully this helps, feel free to ask if you have some more questions. I had mine doing a few different functions and struggled for a while working it out.
Edit- I forgot to mention, when you are setting up the outlet you can set it so it only functions during certain hours, like when your lights are on. You can also set it so if pH drops too low it sets off an alarm.