Mesh Collider

I have a character, and added a mesh collider (cylinder from 3ds max).
I placed the char inside of it, but when I click play it falls through it…
how can I make it stand on the floor of the cylinder (mesh collider).

I guess you place the character “inside” of the cylinder?

Mesh colliders respect the normals of the object you apply them to. Invert the normals in 3D max and make them direct to the inside. Reimport the model in Unity and your mesh collider will stop objects penetrating from inside to the outside.

If you want bidirectional colliders you should probably remodel you cylinder and make the walls two-sided.