Objective Scoring

This my seem obvious or self evident to some but I don't understand  how the Objective scores are calculated? I just joined a game at the end, the one where  as a Zombie, you have to defend the rocket batteries. I staked out the area where the batteries were, defended it for a while and -  zero Objective score. I went after the plants with the batteries and vanquished one, vanquished others and - zero Objective score. I can see the Objective score calculated when you push a payload, but the rest I don't get.  If anyone can enlighten me I would appreciate it. 

thanks.