You can’t normally activate abilities of cards in graveyards. Those that you can activate are those that instruct you to move the card out of a graveyard, or those that specifically say that you can activate them if the card is in your graveyard.

Activated abilities contain a colon. They’re generally written “[Cost]: [Effect].” Some keyword abilities (such as embalm) are activated abilities and will have colons in their reminder text.

Any untapped Zombie you control can be tapped to pay the second ability’s activation cost, even one that just came under your control.