Ok, so for this I am just gonna say that you plugged the solenoid into PC4 Number 1 Channel 2. I am at work so bear with me trying to remember the menu flow.
1 - Press Out on the RKE Head Unit.
2 - Press Enter to select PC4 number 1.
3 - Arrow down to Channel 2 and press Enter.
4 - Press Enter to go into Channel Mode.
5 - Change it to Auto.
6 - Save and go back to the menu you selected Channel Mode on.
7 - Now arrow down to New Function and press Enter
8 - The function you want is controller.
9 - Then the Device is pH (you will need to select the proper SL1 or SL2, when you arrow to one the light should flash on it to indicate which one you are on)
10 - Set Point - arrow till you reach your desired pH level for the reactor. Most likely around 6.8
11 - Hysteresis - I would leave this at 0, if you change this to say .2, this means that your pH could fluctuate .2 above or below the set point.
12 - On When - set to above.
13 - Make sure you save or all will be lost when you back out.