First, just to make sure you have the key, you get it here, in a place called the Southern Shrine:
Then, go out of the maze you came through to get here. From that maze, after exiting up one screen, go LEFT, UP, and LEFT again, passing through all of the rocks and such. You'll see some steps going into the water to the SOUTH. Go down the steps, and swim down one screen. There's an isle with two armos knights. Wake up and kill the one on the left, then go down the stairs. Once your down there, go across the water, LEFT one screen, and use the hookshot to get across the gap. Then go up the stairs at the top of that screen.
Once your out of the cave, you'll want to go RIGHT and UP one screen. Put the Face Key in the slot and unlock the dungeon. Then, go back DOWN one screen, up the stairs on the left, and into Level 6: The Face Shrine.