The old cabin is where your journey begins just after you enter into the dreamscape.
They you will be told how you found it with Amelia in the fields between your houses. She thought of it as a safe place agsinst her father but you thought of it as the untimate club-house. That is where your dreams of becoming a pilot were born. There too, Amelia dreamed of becoming a ballet dancer. You promised to take her away to the big city after you could fly.
To enter the cabin you must first find the key. Amelia said that you put it in a place only you two would think of looking. That place is the old hallowed out tree outside the cabin. The key there will grant you access to the secrates inside.