The Olympic Rainforest diorama is in the "Boettcher North American Wildlife Hall" on the 2nd Floor.  There's an elf peeking from behind the tree with his hand on the tree. He's wearing a green cap.

View standing in front of the Olympic Rainforest diorama.  Elf is peeking around tree.

Slightly zoomed. The elf peeking from behind the tree with his hand on the tree.  Wearing green cap. This is one of the easier ones to find.

Closeup