You pull to open the dump valve, push to close.
In my '75 Argosy24, the pump is just to the street side and aft of the freshwater tank. The sink is on that side on mine, and I have the optional dinette so the pump is under the dinette sink. You should be able to follow the pipes to find the pump or where it belongs. My pump switch is on the wall above the galley sink, just a small unmarked plastic rocker switch.
Il Carriaggio — 1975 Argosy 24 | Il Progetto — 1976 Argosy 28 Center Bath | WBCCI# 15566
He has all of the virtues I dislike and none of the vices I admire. — Sir Winston Churchill