By public bus: 

From Managua take an "Expreso" bus at "El Mayoreo" market towards Matagalpa (2 hours) or a "Local" bus (3 hours). When you arrive in Matagalpa, take a taxi to the "Guanuca" bus Terminal (north)  and there you can catch a bus to "El Cuá", "Bocay" or "El Bote" (important to know that the last bus  from Matagalpa leaves at 1:30pm).

These three buses will take you to "El Macizo de Peñas Blancas" (2.5 hours ride).  Get off at the Km 194.5 and walk 600 meters along the dirt road inside the Macizo. You pass "Don Chico’s farm", the "Cooperativa Guardianes del bosque", and on the right side you will see Reserva Los Nogales. 
​