Multiple textures on a detail mesh

Hello,

I need help on a problem that puzzles me since the previous week:

I bought the Sue Blackman's book "Beginning 3D Game Development with Unity" and i had to add some rocks in the map with some other details.

Now that's the problem, the single mesh brought in the editor as object is perfectly fine, but when i try to import it as Detail Meshes (Vertex lit) the rock has the normal texture + the other detail mesh texture + the grass texture in it.

I'll post a screenshot: