on the ZD3 series you have to remove the front filter and find a small hole below the center thread. You have to stick the blunt end of a 3/16 drill bit into it, turn the valve manually until the bit goes through the "shaft" (valve) and then you "adjust" (bend) the hand-actuated lever so that it stays in the 'engaged' position when you put the pin back in. If that adjustment is "off", much, the pto valve hole won't line up quite right and can contribute to pto clutch failure-which we do NOT want. I know a ZD3xx series is different than ZD221, but I wanted to mention it because the operation is very similar.
Look at your parts diagram. Diagram # C42300. #120 is your pto valve. That "shaft" you speak of, IS the PTO valve. It rotates in the transmission case. There is a hole in the valve. When the shaft is rotated the hole lines up with a set of passages inside the case, which allows pressurized oil to flow from the pump to the pto clutch.
On the ZG series and the ZD221 may be the same, the valve won't come all the way out, just slide it as far as you can get it and clean 'er up as best you can, clean it, and reinsert. IIRC there's a snap ring on the opposite end that needs to come off so it'll slide out.
