You should just have to single left click on an equipped item with a charm quality to open the charm window. The charm slots in Beyond Divinity look like the closed slots in Divine Divinity, but you can still drag and drop charms into the window to permanently add them to the equipment (ie no skill is required to do so). If the charm window is opening on top of the inventory window, just move it off to the side before trying to drag and drop charms, or the inventory window will cover it (move to the top) when you click on a charm.
see also:
charm effects[color:"orange"]Or is this not a technical question ??[/color]
It could be considered technical (or more accurately Help/Tips/Tricks), but is no longer about the patch.
