"Batting zero" is a baseball expression. It means that a player (the batter) has been unable to hit the other player's (the pitcher's) throws with the bat. Therefore, the expression means that Mary has been unsuccessful or inefficient at her job. "To get on the ball" means to start doing better what you are supposed to be doing. So,"get her on the ball" means essentially, "tell her to be more diligent about her assignments [or work]."