The waterfall is located far north of Ardougne, near the Barbarian Outpost. To access it you will need to have Glarial's amulet, which is obtained during the Waterfall Quest, and a rope. To enter, simply board the raft north of the waterfall, use your rope on the rock just down the river from you and then again on the dead tree. Finally 'open' the ledge while wearing your amulet.
The waterfall dungeon is a popular place because of the large number of Fire Giants. If you are going to the waterfall to fight these monsters, be aware that there will usually be a crowd trying to get at them. For more information on this area, visit the 'Waterfall' quest guide.