I've been trying to use the landscape editor to add grass and trees to my cave and when I try to drag grass out into the render window I get the red circle with a line through it tell me that I can't do that.
What is the method for landscaping in cells, rather than worlds?
From what I understand, The grass in the landscape editor are painted onto the ground, interior cells use objects to replace the ground so they can't be painted onto.
You cannot use the landscape editor in interior cells as there is no landscape to edit! If you want grass and tree's, add some of the fern objects, and then the pine tree's. That's your only choice. Unless your cave is big enough and you make it an exterior worldspace that's 'faking' being interior. (Like Blackreach)
Sorry for many posts but, to make what you want, you would probably have to create a "Small World" and build the cave there, use the land mass for adding grass. though this will probably change the way you're cave is.