Hacienda Merida, where I was staying when I visited, highly recommended by the way, can rent you bikes for the 2.25 mile ride to the entrance or even set you up with horse rentals. The walk is very enjoyable too and takes you past local homes and also a wonderful place to have lunch on the way back called Finca Mystica.
From the entrance, Omotepe Biologica' Estacion noticable by the bright yellow gate on the left hand side, pay the 60 cordoba ($3) fee and head on up. The signs say 3k to the falls but its a bit more. The first section is on gravel road through fruit trees and lava rocks. Once you get to the parking lot you will see the hydro-electric building and the trail on the right leading into the jungle. You eventually work your way across a streambed and up a steep climb at the end till you reach the falls. 130 meters high. Take a dip!More