Unity 5 non-convex mesh collisions

I guess you'll have to use an earlier version if you really want to do this, but... you probably really don't.

In general, you do not want to put non-convex colliders on dynamic objects, that puts a lot of strain on the physics engine. (Calculating collisions on moving non-convex shapes is just really expensive). If you have some dynamic non-convex shape, you should approximate it using primitive colliders. (You should also do this with convex shapes in most cases).

Use the NonConvexMeshCollider script to solve your problem: http://www.productivity-boost.com/DownloadNonConvexMeshCollider.html I implemented that by myself because I had the same problem.