once you have the red and blue dildos you gotta find a "secret" signpost that takes you to the right area. from the starting room with the chests take the second path from the right (circled in white) and follow it to the end (white "x"). pay attention to a rock near one of the pages, you'll need to use the signpost under it after you get the last dildo in order to reach the upper left of the cave maze region