1) How do you define areas and the MeshParts (in the editor)?
Can you define it in the code?

2)In a MeshPart of an area can you add a second material in code?

3)How do you obtain the Mshb of the terrain - area?

Noone knows?

I think maybe your questions are a little too broad. Perhaps you should start with searching the forums and reading to get a better understanding first. Then you can post more specific questions. This sort of stuff (that you are trying to do) has been discussed many times before. I just did a quick 30 second search and found a couple of posts to get you started.

http://www.esenthel.com/community/showth...p?tid=6284

http://www.esenthel.com/community/showth...p?tid=3834