Stuck at inventory "puzzle".

[TransgressorOfTheUnspeakableWord]

I'm stuck at the ffirst puzzle, where I actually have to get something (a key) out of my inventory to use it - and it's not oil or a tinder-box.
I tried clicking the environment object (door) which needed the key (expecting a flag to have been set which automatically registered that I had the necessary tool for the operation) to no avail.
I've tried moving the object to various parts of the inventory screen.
I've tried holding the object with the left mouse button and tabbing out of the inventory on the off-chance I may still be holding it.
I've tried looking for a magic key combination under controls.
How do I solve this pointless puzzle, so that I may PLAY the game?

[TransgressorOfTheUnspeakableWord]

It's DOUBLE-click!
Perhaps obvious for those who leave their OSes in default condition, but I'm a double-click hater and it's one of the first remedies I perfom on an OS: hover to select and SINGLE-click to open.