Porque Dumbledore deu-lhe instruções. Esta é a passagem exata à qual você está se referindo:
Harry thought that Dumbledore would insist upon accompanying Riddle, but once again he was surprised. Dumbledore handed Riddle the envelope containing his list of equipment, and after telling Riddle exactly how to get to the Leaky Cauldron from the orphanage, he said, “You will be able to see it, although Muggles around you — non-magical people, that is — will not. Ask for Tom the barman — easy enough to remember, as he shares your name —”
Half-Blood Prince, chapter 13 (The Secret Riddle)
Presumivelmente, as instruções de Dumbledore foram suficientemente detalhadas para que Riddle encontrasse o caminho dele.