It all depends of when they got it.
If before 1942, they win the war, but too unreallistic.
If in 1942-43, they bomb Moscow, or Leningrad, or some concentration of soviet troops, maybe at Kursk.
If in 1944, I think they would probably attack London, or Normandy Beaches if the invasion has already begun. After Bagration, the soviets won't stop just by being nuked once or twice. But the western allies might agree to a kind of peace to avoid nuclear destruction of England. And then the germans could concentrate on the east.
After the bulge, I doubt any german nuclear attack would do more than delay their defeat by some weeks.
But in the autum of 1944, the nuclear destruction of London would certainly paralize allied advance, already stopped by supplies problems. A lot of key figures in the allied staff would probably die, too. If combined with a Bulge style offensive, the western front can be severly damaged. If the britihs are convinced that four or five of their cityes are going to be leveled before the war ends, they migth agree to a truce, specially if Churchill is not around.