You can hold left mouse button for a coin-interface action menu (use, pick up, open,...). While many actions can be performed by just using double click and right click, some specific actions must be triggered by specific commands from the coin-interface (at least the first time exectuing them, after that you can trigger them by double-clicking).
There is a short explanation , the part in the video at 1m40s.
It might have not been that obvious since I didn't use text, but symbols, to explain (as I wanted it to work for both English and German language settings).
There's also a "How to play" description at the info screen when starting the game.
