I have read that the Allies worked hard to confuse Nazi Germany about the execution of D-Day. How did the Nazi high command react when what the Allies were doing became apparent?

German high command and in particular Hitler had convinced themselves that the main attack would be the Pas de Calais area. Logically, this was the best place for an invasion - the beaches made good landing areas, it was also the closest shore to the UK. The Allies decided however, that this was too obvious and the area would be heavily defended. Also, the painful lessons learnt at Dieppe showed the difficulty in attacking a port - these factors led them to choose Normandy.

The Germans assumption was further backed up by the deception plan (known as Bodyguard as a whole, which included Operation Fortitude (South) ) and the misinformation that Agent Garbo (Alaric to the Germans) spread - even on D-Day and immediately after he was still providing "intelligence" to the Germans that Pas de Calais was the real target and the Normandy landings were a feint. For his service, he was awarded the Iron Cross in late July 1944.

Garbo's information was part of the reason that German troops and Panzer divisions were held in the Pas de Calais as late as August 1944, ready to repel the invasion that would never arrive.

Clearly, at least in Hitler's case the immediate reaction was scepticism. For Rommel and other commanders, deperate frustration - they were railroaded in to poor strategy by Hitler's poor decision making and felt that they were being prevented from winning the war. Of course, by the time of the landings Rommel only had 5 month to live, implicated in the assassination attempt on Hitler he would commit suicide in October 1944.