The item needs to be equipped to be able to left click on it and open the charm slot window. Try raising the game resolution; the charm window may be opening off screen (though that usually only happens if you had opened a charm window earlier playing at a higher resolution and then lowered the resolution, or exploited a bug in the unpatched game that allows you to open multiple charm windows). See the link above for details, or this version;
Enchanting weapons/armour/items 
