If you have a scroll or spell avaliable to your character, you need to go to your inventory (Right Click) and there will be a menu there that lists all magical equipment and spells you can use.
Scroll down to the bottem and you should see a list of your equipment. Select the item you wish you use (Left Click). Exit out of your inventory.
Next to your Current Weapon Slot you should see an icon of the item you just selected. Hit "R" and Left Click to cast.
If you're using offensive magic that works On Touch, you need to be right next to your opponent.
