Hi there, @David_Ottergear. Unfortunately, Edmund's chest is in Greenshade Caverns, where we aren't able to go because of the crazy Fog level glitch.
Still, though, you can see the picture of where it's located on an article of my blog, Your Illustrated Guide to Aetherlight Trading Adventures. Just scroll down to where it says "EPISODE 2", and then scroll a little more until it says "THE KEY!!!"
Here's the link to that blog post: