@OwenReynolds has posted an answer to this question Here : http://answers.unity3d.com/questions/238096/terrain-detail-mesh-painting-vs-blender.html

Information :

Detail mesh painting is part of the official Unity Terrain system. For example, you can only "paint" and blend grass/dirt/rock textures on Unity Terrain. You can't do that for a cube that you made. Likewise you can only add wavy grass to official terrain, not to a ball.

A terrain mesh from blender is still just a mesh, so you can't use any Terrain tools from Unity on it. You could maybe convert the mesh to a heightmap and import that as Terrain. But, it's probably going to be better just to remake your terrain from scratch, in Unity, with the Terrain type. It works fine once you get some practice, and will probably give better results in the long run.