The secret storage area is on the second level of Hogmarsh Hollow, in the mission where you have to recover the king's treasure, Swamp of Eternal Drench. Around 1 screen from the start of the stage you will come to a T junction with a path leading top left. Follow the top left path and you will come to a hatch in the ground on an island surrounded by swamp water, press Y to enter the hatch and get the achievement.