This achievement is story progression related and cannot be missed. After completing the mansion in the swamp, defeating Varluk Drooh, and rescuing Frogboy, you'll receive access to the frog statues that are found all over Umbra. Frogboy himself will tell you to interact with one after a short dialogue scene. Doing so will land you on Plumpet Island and should reward you with the Gersween's Nest achievement.