There is a small circular reticle (crosshairs) in the middle of the screen; point that at the key. It should be enabled by default, and easy to see if you are looking for it (and not hard to miss if you are not), but can be disabled in the Gameplay section of the options.
The chest contents are random. You can quicksave before opening it, but I wouldn't waste too much time reloading.
In the meantime, there is something else to find in the training area.
