The option to give a darwin requires a minimum amount of posts, I think it might be either 50 or 100. At that time, the green and rec icons will appear below users avatars and can be clicked on. Probably the best way to test this is to keep an eye out for the icon as your post count increases, then find one of my posts and try hitting the green icon to make sure its working.
I've made this suggestion to a few newcomers, and one day one of them will reward my efforts by doing it. I anticpate you will be this person, and to show my appreciation I have composed a short verse:
Ruby Leo, thank God she joined
The phrase 'lurker' after her was coined
She hung out with some lutherans and occasionally a baptist
if you give me a karma I'll make a video and rap this
She home schools her kids, she hasn't yet said why
I'm hoping that she will because it's something we might try
She went crazy in college but has not elaborated
by this Australian theist shes been 'versed'...but not debated.