A. How do cellular concentrations of ATP and ADP regulate glycolysis (speeding up or slowing down)?

WHen an excess of ATP is present in the cell, feedback inhibition will occur. It will bind to enzymes early in the glycolytic pathway, inhibiting them. This occurs because the cell is already "full up" in terms of energy levels, and doesn't need to make lots more -- why waste cellular resources to make something that you already have? So, the high ATP concentrations already present in the cell will feed back, inhibiting earlier enzymes.

On the other hand, when/if ADP is present in the cell, this is likely due to the fact that ATP has been hydrolyzed -- ADP is the product of this hydrolysis. So, ATP levels in the cell will be low. Therefore, none of the enzymes in the glycolytic pathway will be inhibited by cellular concentrations of ATP, because cellular concentrations of ATP will be too low to bind to the enzymes.

B. In the body, when is pyruvate converted to lactate?

Pyruvate is converted to lactate when the muscles are lacking oxygen, and respiration must occur anaerobically. This most often occurs during exercise which is so vigoorous that the body's demand for oxygen is greater than the rate at which you can supply it through breathing. (This can only be sustained for short amounts of time.)