well your object turns gree because mental ray doesn't have a hook into maya's hardware display function. That is normal and it's OK. If you really want to see a color, you can do the folllowing.

In the Hypershader, graph your MR shader node. You should see a shaser group node (SG node). If you look at the attributes of the SG node, you'll see that your MR node is going into the "Material Shader" attribute. No create a blinn shader and map it into the "Surface Material" attribute of your SG node. Now you model will not be green anymore. This will have no effect whatsoever on your renders, it's just for Maya display.

As for why your renders are turning out black, I have no idea. You'll need to tell us more about what it is you're doing. Maybe you have no lights, or maybe you're renderer is set to Maya software and it can't render the MR shader. Maybe you've got primary visibiltiy turned off.

I hope this helps.
-dann