Thread:NamelessRumia/@comment-17228848-20151224054404/@comment-26521006-20151224055405

That's because you need to use the Mystery Key in the item menu while in front of the door. The door doesn't open immediately when you interact with it.

https://40.media.tumblr.com/bd26e8d1a3e10edb6f3495d05e42f0a3/tumblr_nzuloo0vNL1rx4pnso1_1280.png