Character falls through floor any suggestions?

Hey guys I'm VERY new to Unity3D, and I'm having a little issue. So I downloaded some free 3D Models from the Assets Store. I'm using one called "Robot Kyle" basically when I add him to my work space I add a Rigidbody to him and use gravity for him and place him a decent margin above the box I'm using as a floor, however the character goes right through the floor. As a test I placed a small cube with a Rigidbody also and it hit the floor perfectly. Can anyone explain what I can do.

P.S I've tried using a box collider on the character which slightly worked, but the physics would clearly be off.