Way in is mostly up-hill. Ride in for the first 4 miles, then park your bike and hike the last 2 miles to the falls. Ride back is all downhill.
From the Laie Hawaii Temple, go right, and when you get to the round about head down the road directly to the left of you. Follow this road down through the gate and you should see the trail sign on your left.
http://www.singletracks.com/item.php?c=1&i=3798