I have trees on my terrain that were generated through the tree brush tool. I have “Create tree colliders” checked and my trees all come from a prefab with a mesh collider on it. However, you can still walk through the trees. If I change the collider on the trees to a capsule collider, it works. Also, if mesh colliders simply aren’t accepted, which ones are?
Only “convex” mesh colliders block/collide with ridigbodies (i.e. your player character).
You can check the “Convex” option to enable it.
http://unity3d.com/support/documentation/ScriptReference/MeshCollider-convex.html
You should use convex Mesh colliders only when you need precise collision detection, otherwise use the primitive ones as they get you better performance