The problem is, there is no definitive guide, "Do that, get merit". Basically they're telling you,
"go ahead, come up with your own idea how to get merit on our site". You cannot even buy it directly, only informally. I believe they could at least make certain manageable tasks so that people could get merit for doing something.
Yes, I earn merit by helping to moderate a few topic of the Russian section of the forum.
But you are not telling how you became a moderator.