i am currently trying to upgrade my manual co2 system (CO2 regulator with precision needle valve +cup diffuser) into a automatic system like the one at petwarehouse. since i have never seen a solenoid valve before, my question is: how to i fit a sparate solenoid valve onto my current regualtor?

You screw in to the outlet of your regulator. The solenoid be the last gate keeper of CO2 release. The solenoid will only open to allow CO2 release when it's power is on. So to use the solenoid, you will have to plug the solenoid power cord to a pH controller or a timer.

BTW, there is no dumb question.