From Managua airport get a taxi to Mercado Roberto Huembes, from there you can either get the bus directly to Las Salinas and get off at Las Mercedes (keep in mind there is only 1 bus per day at 3pm) or eventually you can get a bus to the city of Rivas and from the same bus stop in Rivas get a bus to Las Salinas and again get off at Las Mercedes and walk 600 meters to the hostel.