If you click on an equipped item with a charm quality, a charm window will open. If necessary move the charm window so it is not over the inventory window, and then just drag and drop charms from your inventory (etc) to the charm window to permanently add them to the piece of equipment.
Do you have the game paused when you are trying this? I never tried charming anything that way, but you can not drag and drop items out of your inventory when paused.
charm effectsWelcome to the forum.