The solenoid is an electrically operated valve that you generally plug in to a pH controller such as Milwaukee SMS122. You set the controller to turn the solenoid on as pH goes up, and off as pH goes down. Opening the solenoid allows the CO2 to flow across the media, giving you your calcium. Generally the effluent from the reactor is dripped into a cup, and the controller probe is set up in the cup as well. That way the controller is only reading the pH of the reactor effluent and most people seem to set their control band around 6.5 (solenoid off) to around 6.8 or so (solenoid opens). Then when the cup is full it overflows into your sump. ;l
HTH,
Mike
<EDIT> Just realized I didn't answer your original question about a regulator recommendation. I can't help you there, I am using the one that came with the used MRC reactor I picked up.