Zacama's triggered ability triggers if you cast it from any zone. It doesn't trigger if you put Zacama onto the battlefield without casting it.

Zacama's damage-dealing ability can be activated during combat, perhaps targeting a creature blocking it. If all creatures blocking Zacama are destroyed, its combat damage is assigned to the player, planeswalker, or battle it's attacking because of trample. If the blocking creatures are dealt nonlethal damage, that damage is considered when assigning trample damage.