how do you nuke it?
is it just a simple nuke or you put a damp paper towel on it?
Yeah, you need something wet in there to produce the steam. I take a big tupperware-type container, put wet paper towel in the bottom with whatever I'm steaming on top of the towel, then put the lid on leaving one corner popped open. Nuke it for a few minutes - enough to get the steam going.
also do people normally eat the green stuff inside the main shell?
Depends. It's edible, but it's also the part of the body where any toxins the crab has encountered will accumulate, so it's your choice.