Lushoto is reached from the main Dar–Arusha highway: get off at Mombo and take a dala-dala (shared minibus) or taxi up the switchbacks for the last 32 km — the climb is steep, slow and one of the most scenic short drives in Tanzania.
From Dar es Salaam it is about a six-hour drive plus the hour up from Mombo. From Arusha it is roughly four hours. Long-distance buses from Dar will set you down at the Mombo junction; check that the bus stops there before buying the ticket.