UV unwrapping

I have a 3D model of a sci-fi tank. I want to texture it so I apply a Unwrap UVW and then open the editor and

select the object, but when I go to mapping > flatten mapping, it is greyed out. It is like this for every object.

Also, once I have unwrapped every object in the scene what do I do then?

Thanks

Hello heartscool.
What software you are talking about, 3ds Max?
After you have unwrapped start applying materials with textures.
You can save the UV map as an image and use it as a guide in Photoshop when creating textures.