Physics: mesh moving on top of of another I'm not sure how to use physics to make a mesh move on top of another, lets say a ball rolling on an uneven surface. 

I'd like to be able to keyframe the moving object from point A to point B on the X and Y coordinates, but to have it follow the uneven surface of the Z axis.
If you really do not need the Physics simulation, you could use a Shrinkwrap constraint. To use a shrink wrap constraint, Add an empty, then  parent the empty to the ball (Ctrl P). Then add a Shrinkwrap constraint to the empty. Set the Shrinkwrap Target to the ground. For your case, you could set the Shrinkwrap Type to Project, then set it to -Z.



Keyframe your ball. Set the ball as a active rigid body (Add Active), in the Physics tab of the Tool Shelf. Then on your ground plane set it to a passive rigid body(Add Passive) in the Physics tab of the Tool Shelf. Play the animation from the beginning Shift Left arrowto jump to the beginning, and Alt A to play.