The drain and fill pumps will be on the port side of the boat below the motor. They will be mouted to the stringer just below the motor. I would suggest removing both the lower and upper access panels to get to it. It's not that big of deal to take them out.
If the system is factory and hasn't been modified, there will be one 750 attwood aerator pump to fill and one to empty.
The routing of the stock ballast is as follows:
The water will intake through the thru hull go through the fill pump and push water through the drain pump in reverse. Then the line tee's off and goes to each tank. On the top of each tank near the front, there is a vent that runs to the seacock valves you were asking about originally. Those seacock valves basically put an airlock on the tank so the tanks will not take on water while driving if the valve on the thru-hull is open.